Maybe Mac OS X 10.8 will be White Lion.
I don’t think I’m not breaking any NDAs by saying this (since it’s all over the web and a lot of it is on Apple’s own Lion Page, although the more interesting stuff is on the Developer version) but 10.7 looks like a huge upgrade over Snow Leopard — a bigger upgrade than Leopard was over Tiger. What are the killer features? Well, not the UI frippery. The interesting thing is not just how deep and powerful all this new stuff is, but how interdependent it is.
Versions. Lion’s file system is version-controlled. Every time you open a document, every hour, and every time you save, a new version is recorded. This goes beyond Time Machine because (a) it doesn’t require a Time Machine volume to be around, and (b) Time Machine saves versions of files at intervals, so multiple saves within an interval (which can be quite long for a MacBook, say) are going to be overwritten. What isn’t clear from just reading the summary (I’ve not yet downloaded and installed the preview) is how much of this comes “for free”. (If it does require apps to use the new API, the API is very simple, you override a method to return true.) It could be pretty nasty to automatically version control giant video files, but it would also kind of suck if it only works for new versions of apps. I suspect it works for everything by default; apps can definitely fine-tune their behavior (much as with Time Machine). Either way, it’s a huge feature.
(Versions apparently depends on another new feature called File Coordination which is designed to help prevent multiple processes from corrupting files by serializing file access among processes. Aside from anything else, this deals with the obvious use-case where you have the same file open in two different apps. I haven’t highlighted this as a major feature in itself since I don’t really understand it.)
Autosave. Because saving is non-destructive, saving automatically is a no-brainer. Lion saves all your work automagically. The idea that this has been brought back to the Mac from iOS might be good marketing but it’s disingenuous. Most iOS Apps have destructive save behavior — in fact it’s quite infuriating with some. iOS doesn’t do anything to help — the only thing that might help is that Apple will probably reject submitted apps whose behavior is too pathological (but this is not dependable based on my experience).
Resume. Lion remembers an application’s state between launches. And because everything is saved automagically and the OS remembers App state, it can quit apps to free up resources whenever it wants to. OooooOOOOooooh.
Overlay Scrollbars: forget everything else that’s been added to the UI, scrollbars are no longer ugly or a waste of space.
AV Foundation. Just as Core Image allowed any talented programmer to whip up a reasonably decent Photoshop-wannabe and Core Animation allowed every indy developer to turn prosaic source control front ends into UX masterpieces, AV Foundation will allow anyone with the inclination to whip up a non-linear video editor. Expect at least two or three Pixelmator-quality video compositors to appear within the next six months.
Security. Apple is providing developers with tools to intentionally sandbox their applications (“Sandboxing and Privilege Separation”). If your app doesn’t need to do file i/o you can let the OS know and even if it gets infected by malware it will still be sandboxed. This is probably not a huge deal for third-party developers, but chances are Apple will use this functionality extensively in its own apps (e.g. Safari) and strongly encourage certain other software vendors (Adobe, Oracle, Microsoft) to do likewise. Given that the common “surface vulnerabilities” of OSes tend to be applications written by a few very large development teams this gives them tools to make their software much less vulnerable.
Other sites have noted that Filevault has been improved to support full-disk encyption, which should be more useful and less buggy than FileVault is right now. Mac.blorge points out that 10.7 includes full malware protection, Google Safe Browsing in Safari, and improvements to ASLR.
Mac OS X Server merged into Client. This may be huge — I don’t know. But the basic premise seems to be that you can have as many of Mac OS X Server’s features running on your Mac as you want or not. Whether this will prove as convenient as, say, running MAMP remains to be seen. It sounds nice though.
Full Multi-User Support. You can remotely log in as a different user while your Mac is being used. Effectively, a Mac is now a time-shared system. (Should make for some really interesting Hackintoshes.)
Air Drop. Convenient system for sharing files locally with zero setup.
As for other stuff. Well, the new UI widgets will probably get used like crazy for a while and then fade into the background. (Maybe folks will finally ditch drawers and use popovers instead.) Springboard looks nice, but then it’s replacing Application Folder Chaos. The full screen app stuff, Mission Control, new gestures — the devil is still in the details. Spaces seemed huge but ended up being a dud. Expose I use occasionally. OTOH I frequently accidentally zoom things with the unintentional pinch gesture which overlaps too much with the right-click gesture. A lot of the gesture stuff is already in Snow Leopard anyway and I don’t see many people using it.
Oh yeah, 10.7 adds TRIM support for SSDs. (TRIM support can improve SSD performance and longevity. Don’t worry though — with normal use SSDs will be just fine without it.)
Finally, 10.7 drops Rosetta support. (Rosetta is the PowerPC emulator that allows pre-Intel apps to run on Intel Macs.) I don’t know about you, but I haven’t run anything under Rosetta for ages. Dropping Classic support was a much bigger deal since there are still useful apps that never made the OSX transition. Recompiling an OSX app for a new CPU architecture is a much easier exercise than porting from Classic to OSX (and then there’s apps that had been orphaned long before OSX came out, like Studio/32).
So, Lion looks like a must-have upgrade, but I doubt it will be $29.