All in all the big news for the operating system is the integration of search and metadata technologies into the heart of the Operating System. The Safari RSS and iChat AV stuff is pretty cool too and everything else looks like tweaks, gimmicks or outright rip-offs.
I would also echo Tom’s thoughts on Launchbar (another Tiger rip-off) with my feeling about Quicksilver. Quicksilver feels completely at one with the OS and I use it all the time without thinking: command-space, f, w to launch Fireworks for example. Incidentally, Quicksilver has just been updated to beta 25 with some architecture changes and bug fixes.