In a few months, you won't have any choice but to upgrade to Windows 10—unless you want to lose security updates and support.
The Windows 7 faithful have nothing to fear, though; Windows 10 isn't much of a paradigm shift from the now-decade-old OS. And in my experience (and that of many I know), it's been pretty delightful to use. Things like Windows Hello face login, touch-screen support, multiple desktops, Quick Access in File Explorer, and even the Edge browser improve my daily computing.
Sure, those few souls who depend on Windows Media Center for their home entertainment will be disappointed to find it gone in Windows 10, and some may be running very ancient software that's not compatible with the updated OS. Privacy is another issue, but you can disable usage reporting (though you lose some of the best new features), and you don't even need to sign into an account.
The key takeaway, however, is this: In most of the things that really matter—speed, security, interface ease, compatibility, and software tools—Windows 10 is a massive improvement over its predecessors.
A major thrust of Windows 10's design was to be more familiar to users of pre-8 versions of Windows and more attuned to use with a mouse and keyboard than Windows 8, with the return of the Start menu (albeit in modernized form) a major part of that effort. Nevertheless, Windows 10 is a chance for everyone who missed out on all the real performance, security, and feature advances in Windows 8 and 8.1 to get caught up.
Windows 10 has now seen seven feature updates since its initial release in 2015, all of which, like those for macOS, cost nothing. Even the updating process has been improved, with more control over when they take place. You can even delay updating Windows for an extended period if you prefer.
The operating system was a free upgrade until summer 2016, but now that party is over, and you'll have to pay if you're still running earlier OSes. The Home edition lists for $139.99, and the business-focused Pro edition goes for $199.99. Hunting not very far around the internet turns up greatly reduced pricing, however.
If you've already made the upgrade to Windows 10, all subsequent updates are free, and if you buy a new PC, it will certainly come with the new OS. You can also purchase USB installers for both editions at a physical Microsoft Store or on the Microsoft Store website.
At first launch, Windows 10 didn't serve tablet users' needs well, but that has changed. For example, the Edge web browser now lets them swipe back and forth through browsing history, and the All Apps view on tablets now fills the screen, rather than being a very deep list. Of course, the vast majority of update candidates are running existing Windows versions on laptops and desktop PCs, and for them the upgrade, as the list below will bear out, is highly recommended.
Note that Windows 10 has shed features that weren't used by a large enough audience for Microsoft to continue offering them. Things like the aforementioned Windows Media Center, Windows 7 desktop gadgets, and a few more trifles bit the dust at first launch. The most recent Windows 10 update, the May 2019 Update, removed a half dozen more components that are even less widely used.
An underlying reason to upgrade, aside from any features listed here, is to take advantage of advances that come with newer technology; simply experiencing the little conveniences and updated design that accompany any new operating system can be refreshing. Several of the advantages listed below were also features of Windows 8.1. But a major goal of Windows 10 is familiarity and a much simpler learning curve than Windows 8. From that standpoint, Windows 10 aims to give you the best of both worlds.
Without further ado, here is why you should upgrade that old Windows box: this list highlights some of reasons you'll be thankful you updated.

1. Speed

Startup and more. If you never made the move to Windows 8 or 8.1, you've missed out on one of the best things to hit Windows operating systems in forever: Fast startup. There are even comparison videos showing that Windows 10 starts up faster on a MacBook than macOS. And the Mac's operating system has long been remarkably fast. Compared with Windows 7, the newer Microsoft OSes leave the older one at the gates. DirectX 12, the 3D engine that gets game developers closer to the metal for a new level of immersive performance, offers another speed boost.

2. The Start Menu

Some loud voices in the tech community long clamored for the return of the Start menu after its replacement by the Start screen in Windows 8. Which is kind of funny, since when it first appeared in Windows 95, the same group scoffed at Start as too noob-like. Anyway, Microsoft heeded the cries for its return, but gave it a tile-based appendage, so as not to lose live tile info, and to still allow for the OS to be touch-enabled. Since tiles come in various sizes, you can give the more important apps bigger tiles and the extras smaller ones.
Windows 10 Start Menu

3. Cortana

It's nice to be able to talk to your technology. If you've used an Xbox One or talked with Siri, you know how convenient it can be to interact with your technology hands-free. "Hey Cortana, play music," or "take a note" are just for starters. You can now shut down (or put to sleep) the PC with Cortana—quite useful after a long day of computing. Maybe some day people will come to appreciate the pleasure of not having to type and mouse-click just to open an app, web page, or to get info.
You can get more specific with reminders that will show up on any device running Cortana—including Androids and iPhones. Cortana isn't just about voice commands: Her Notebook keeps track of your interests, popping up info such as your favorite sports team scores, local weather, and even traffic conditions for your commute home. If you have a Cortana speaker like the Harman Kardon Invoke, you can tell Cortana to start, shut down, or mute the console. The same holds for controlling smart home devices like the Philips Hue lightbulbs.

4. Universal Apps

Windows Microsoft App Store
If you still use Windows 7, you don't have an app store. Windows 10 lets you find software you need for large and small tasks, and you can run apps either windowed or full-screen. These apps run in their own sandboxes, so they're more secure than old-school Windows apps. They also integrate with the system by offering notifications in Action Center and built-in sharing; for example, a photo app could share to an Instagram app using a standard share button.
Windows 10 also comes with slicker and more powerful productivity and media apps, including new Photos, Videos, Music, Maps, People, Mail, and Calendar. The apps work equally well as full-screen, modern Windows apps using touch or with traditional desktop mouse and keyboard input. Like the OS itself, these apps are periodically updated with new capabilities.
The Windows Store isn't just for apps, either: You can get movies and TV shows for rental or digital purchase, and even shop for your next Surface computer or Xbox console there. Of course, you'll also find a selection of PC games, several of which can be played on any device you log into.

5. Touch

Just about every screen in your life these days is a touch screen—your smartphone, your tablet, even your car navigation system. So why not your desktop or laptop PC? I've heard the whining about fingerprints on the screen, but why is that more of a problem than on a smartphone, where a fingerprint takes up a much larger percentage of the surface area?
I've used the all-in-one PCs like the Asus Zen AiO Pro Z240IC, the Surface Pro 6 convertible tablet, and a tower with an Acer T232HL, and I can tell you that, while touch isn't my primary way to input to the computers, it can be darned convenient at times. The Start menu tiles and edge swipe-in panels make Windows 10 even better than iOS for tablets in some ways.

6. Action Center

Action Center
Your smartphone pops up notifications for messages, updates, and even breaking news, so why shouldn't your PC? With Windows 10 it does. Similar to the macOS Notification Center, the Action Center shows messages from email, the system itself (you've installed an update, for example), and from apps. You may see a weather warning or a birthday reminder. Windows 7 had a rudimentary notifications feature, but those system tray notifications went away after a short period and didn't offer much in the way of interaction. And if you don't want to be bothered by notifications, Focus Assist quiets them for a spell.

7. A Better Browser

Edge Browser
Microsoft Edge modernizes the OS's default browser. That means improved compatibility and speed, as well as helpful new capabilities like webpage markup and reading mode. Edge has also been shown to be much less demanding on laptop and table batteries than Chrome. The current version of Edge will soon be replaced with one using the Chromium browser under the hood, which will make for better compatibility with websites. We hope Microsoft keeps all its browser's most useful and unique features, including on-page text lookup with Cortana, the built-in sharing icon, and the slick Fluent design.

8. Security

Windows 10 inherits the Secure Boot feature from Windows 8 and makes it even more secure. This requires that any code that runs right when the OS starts is signed by Microsoft or the hardware maker. Unlike Windows 8, Windows 10 PCs can be set up so that this feature cannot be bypassed. Three other security feature for Windows 10 are Device Guard, Microsoft Passport, and Windows Hello. More recently, Microsoft added both ransomware protection and exploit/threat protection.

9. Virtual Desktops and Timeline

For years, some of the more-sophisticated Mac users have found the ability to switch among several virtual desktops useful. Windows 10 finally brings the capability to Microsoft's desktop operating system. It's incredibly easy to use: Just click or tap the task-switching icon next to the Cortana search box in the taskbar. The task switching view now also reveals Timeline, which shows your browsing and app activities to get you easily back, if you allow it, of course.

10. Xbox App

Windows 10 Xbox App
If you're a gamer, you'll love the integration with Xbox that comes in Windows 10. Not only does the Windows 10 Xbox app let you keep track of your friends and achievements, you can also stream games from the console to the PC and play multiplayer games from your PC against other players on Xbox. The Windows Store now eases the process of finding games and purchasing them for use across the PC and Xbox.

11. Game Bar

Windows 10 Game Bar
Yes, the previous tip was about gaming, but the Game Bar—summoned with Windows Key-G (if not, make sure it's active in Settings > Gaming > Game bar)—deserves its own entry. This tool lets you snap screenshots, record screen video, control audio sources, and broadcast your gaming session. It now features separate panels for its various functions: One shows performance specs, including CPU and RAM use; another lets you chat with fellow gamers, and another has all the audio settings. It's also helpful for capturing non-gaming video clips in Windows 10.

12. Better Screen Capture Tool

Hitting Windows Key-Shift-S now lets you select an area for screen captures, whether that's a rectangular area using the cross-hairs icon, the full screen, or current window. Once you've snapped the screenshot, you can do basic cropping and markup, and save or share the result. It's successfully removed my need for third-party screenshot software.

13. Supercharged Copy and Paste Clipboard

Cloud Clipboard
One of the newer features in Windows 10 is one I now use every day: Clipboard History. This lets you paste from multiple items you recently copied or cut (usually with Ctrl-C or Ctrl-X) and works with both text and images. The clipboard is no longer limited to one PC, but your copies can roam to other devices. You can even pin items you use repeatedly to the new clipboard.

14. OneDrive On-Demand Syncing

Microsoft OneDrive
You need to be taking advantage of cloud storage these days, and OneDrive is one of your better options, especially (but not only) if you use Windows. OneDrive lets you keep files in the cloud without taking up space on your local storage, with the Files On Demand option. It looks like a regular folder on your desktop, and also can sync operating system choices like desktop background.
OneDrive files are accessible in your web browser as well as in iOS and Android apps. It does a great job with photos, by the way, showing the photos location in a small map and even offering object identification, so you can find all your photos of bridges or cats. OneDrive also lets you retrieve any files from your PC remotely—if you're PC is on, of course.
OneDrive really comes into its own, however, when you use Office apps: You can have the apps save continuously and available to any of your other PCs and devices wherever you are.

15. Smartphone Tie-Ins

Your Phone App
Even though Microsoft no longer actively develops its ill-fated Windows Phone operating system, new tie-ins with Android and iOS make your phone work in sync with your computer. You can use the Continue on PC app to send a webpage or document to your desktop from your phone. You can use the Edge mobile browser to sync your browsing, and the Cortana app to share reminders among both platforms. Android users get the most benefits, with the Your Phone app that lets you see photos as soon as you shoot them from the phone and interact with SMS messaging.

16. Voice Typing

Simply press Windows Key-H (for "hear") and your PC starts typing what you say—no complicated setup required. Read more on how to type with your voice here.

17. Dark and Lights Modes

Windows 10 Dark Mode
Dark mode has made it to most commonly used parts of the Windows interface, and in the Windows 10 May 2019 Update, Microsoft gave some love to a light(er) mode as well. Navigate to Settings > Personalization > Colors. Under "Choose your color," you should have the option of Light, Dark, and Custom. Some browsers and websites ever respect your Windows 10 dark/light interface choice.

18. Nearby Sharing

Nearby Sharing is a nifty capability that flies under most PC users' radar. This is very similar to Apple's AirDrop, which lets you send a photo or document directly using Bluetooth or Wi-Fi—no internet connection required. It's useful if you need to quickly get a file from, say, a laptop to a nearby desktop, but the capability is not yet supported on mobile.
Nearby Sharing in Windows 10