- Easy install. The simplest (and therefore best) install experience I've seen.
- Snappier. A little, at least -- I haven't really noticed huge increases on anything yet, though. Maybe 10-20%?
- Weirdness on first run. For the first few minutes, spotlight couldn't find any of my apps -- then a few minutes later it could. System Preferences (under the Apple menu) wouldn't launch, then a few minutes later it would.
- Keyboard Maestro/Spaces weirdness. Any of my Keyboard Maestro macros that switch spaces occasionally require me to run them twice for them to actually switch. Not sure what's going on there...
- More disk space! Hmm, where did that extra 10GB come from on my disk? Uh, maybe some of it came from here -- a "GB" represents about 74 MB fewer actual bytes in the count now, so what looks like an extra 10 GB is actually closer to 9 under the old accounting.
- Scanner. Preview can grab images directly from my scanner.
There doesn't appear to be anything else that's really changed.
One thing I'm really excited about (but haven't seen yet) is in ejecting disks -- the old OS would just basically say, "No, this disk is in use..." SL will actually tell the apps to let go of that disk, and if they won't, it tells you which ones are hanging on. Very nice.
Worth $29? I guess so -- though from what I've seen I wouldn't break any speed limits getting to the Apple store.
UPDATE: My mom tried installing SL and in the middle of the install it seemed to crash, so she force-quit and restarted, only to discover that those steps corrupted the HFS+ disk catalog. Proceed with caution, and make sure you have all your stuff backed up (including those painfully large Entourage databases).
Pretty good explanation