Apple made some fairly significant changes in Leopard under the hood (LDAP, CUPS, 64 bit, fully "Unix", X.org X11, etc.), mostly for the better, but it's going to take some time for the kinks to get worked out in the new implementation.
You're probably going to like:
- Time machine: easy backups, and done by creating date-stamped folders on the backup drive with your entire disk structure in each -- their secret is "hard links", which I'll explain another time, but is rather ingenious (requires another connected hard drive; watch out if you use Entourage 2004, your mail database, which for lots of people is pretty big, probably changes every time you get your email, so Time Machine is going to back it up every hour which will fill your backup drive in a hurry)
- Quick look: select a file and hit the space bar to see a quick preview of it -- nice
- Spotlight: is much faster (replaces Quicksilver as my launcher of choice), answers math questions and defines words for you
- Preview: lets you combine pages from .pdf files and save them to a new .pdf -- this is *awesome* if you need to send any kind of compound document to anyone, mix and match Excel plots with Illustrator pages with PowerPoint slides -- love it!
- Screen sharing: as if you're sitting in front of your Mac from almost anywhere, all built-in (see wikipedia on VNC)
- After printing one job on our HP Deskjet 812c printer, the second one comes out as a single line of text across the top of a few pages; we have to shut it off before it'll print right again
- Tried the guest user account, hard-locked up the machine within 30 seconds -- not sure what happened, couldn't even ssh in from my linux box
- Bunches of third-party apps won't launch, who knows why
- A co-worker upgraded and his Finder locks up with the spinning beach-ball of death (SBBOD) every time. He had to create a new user and start copying over all his stuff to keep it from doing that -- hopefully he'll know soon what's causing it
- X11 breaks a bunch of stuff, notably multi-monitor support
- Firewall took a huge step backward, I don't even understand it anymore