Mac OS X has been around for more than 10 years. You’d think that by now there wouldn’t be much room for improvement. But this latest version – 10.7 if you’re counting – would prove you wrong.

Apple says Lion adds 250 new features to OS X. Many are borrowed from iOS. Some are tailor-made for new Mac users. And others improve (or fix) features that have been around for a while. Do these changes make Lion a must-have for you?

To help you answer that question, we’ve assembled summaries of the most important new features, and Macworld’s Jason Snell has written an extensive review. Read on and decide for yourself.

Versions

Time Machine backs up your stuff hourly, but what if you want to revert to changes you made 20 minutes ago? Versions is Apple’s answer.
Versions works hand in hand with Auto Save and Time Machine to make sure significant changes to a document are saved. There are a few ways to access Versions. You can revert to the last saved or last opened version through the File or Versions menu (you can hover the cursor over a document’s title bar to access these).

If you choose the Versions menu, you’re moved into a Time Machine-like interface showing two documents: the current version on the left, and a stack of previous versions on the right. Click on the title bar of an older version to bring it to the foreground, or use the history slider on the right. Click on the Done button, and you’ll return to the application without making any changes; click on Restore and the selected older version will replace the current one. You can copy and paste text from the older versions if you don’t want the whole thing.

Auto Save

Lion makes C-S unnecessary, because the OS automatically saves your work whenever you make changes. Auto Save operates pretty well, but it might be hard for long-time Mac users to trust it. Fortunately, you can still reflexively press C-S if you want to ensure that your most recent version is definitely written to disk. What’s missing is a Save As command, but don’t worry: you can achieve the same effect by using the Version menu’s Duplicate command.

Resume

When you launch an application, it opens right where you were when you last closed it, with all the same documents and windows open.
Resume works not only when you quit an application, but also when you restart your Mac. In fact, when you select Restart, Shut Down, or Log Out from the Apple menu, you’ll see a checkbox that lets you specify whether you’d like windows reopened when you log back in.

When it works, Resume feels truly magical. But – like Auto Save and Versions – third-party developers will need to build support for it into their applications before the feature will be widely available. Apple’s own applications support all three of these data-saving features.

10 little things

Signatures in Preview

In OS X’s built-in PDF reader, Preview, you can now use your iSight camera to scan in your signature; the signature is then made available as an annotation. That means you can drop your scrawl into any PDF form.

QuickTime goes (almost) Pro

If you were disappointed when Apple retired the Pro version of QuickTime, Lion should cheer you up: the new version of QuickTime regains some of QuickTime Pro’s power, including the ability to merge and rotate clips.

Correct and define text

Lion can fix your misspellings or add accents to letters on the fly by using bubbly autocorrect pop-ups – just like those on your iPhone or iPad.

Better System Information

From the Apple menu, select About This Mac and then click on More Info: instead of the old, dry-as-dust System Profiler, you get a new, friendlier System Information screen.

Fancier Photo Booth

The major change here is that full-screen mode turns your entire screen into a carnival picture booth, shimmery red curtains and all.

Simpler file transfers

Open a Finder window and select AirDrop in the sidebar. (Or choose AirDrop from the Finder’s Go menu or press C-Shift-R). Nearby Macs on the same local network that have the AirDrop window open will appear. To transfer a file, drop it on the other Mac’s icon in the window.

Encrypt everything

In Snow Leopard, FileVault could encrypt and decrypt the contents of your Home folder; in Lion, FileVault can now automatically encrypt and decrypt your entire hard drive.

Share your screen

With the Screen Sharing utility, if the computer you’re connecting to has multiple user accounts, you can log in to one of them remotely without interrupting the others. And if you don’t have an account on the Mac you’re connecting to, you can log in with your Apple ID.

Meld iChat accounts

iChat finally allows you to combine multiple accounts into a single, unified buddy list. You can also use a single, unified status message: update your status once, and the same one shows up in all of your messaging services.

Migrate from a PC

Lion’s Migration Assistant can now help users migrate from old Windows machines, as well as from Macs.