Spotlight
Recently, Apple's been promoting the hell out of Tiger in the developer community, especially Spotlight. I think they are a bit worried that they'll throw a party and no one will come.
From what I've seen, it looks like adding Spotlight support to an app is very easy. The real problem is going to be figuring out what to do with that support. Most of the discussions about meta-data seem to center around MP3s (no surprise there) and digital photos, but the examples for the latter seem a little forced. Who wants to sort photos based on aperture? I'm having a hard time thinking of good excuses to use Spotlight's meta-data features. None of my projects is really Spotlight friendly, but I suppose if I ever get spill off the ground, it could definitely be used in that.
Pervasive content indexing, on the other hand, will probably save me hours, assuming someone can trick it into indexing all the sites I visit, all the emails I receive, all the feeds I follow, and all the papers I read (and then lose because they have filenames like pldi-fsdf.pdf). I am looking forward to using the Finder to actually find stuff again (it's been quite some time since this has been pleasant [and for a while it wasn't even possible]).
I also have 4+ years of developer discussion lists stored in Mail.app. If Spotlight can make Mail's search usable, it will have been well worth the wait.