I'll upgrade because I like to keep up to date with Apple stuff.
All major updates of OSX have been really useful and enjoyable for me, although I think I skipped 10.2.
It seems to me like the theme of this update is "workflow".
Besides Dashboard, Automator & Spotlight another thing that I would like to be able to use is the general cross-integration of applications.
This was promised in OSX, but it seems like it's starting to get implemented only in tiger, really.
I mean that you'll be able to add stuff to iPhoto from Mail, or Spotlight or the Finder.
Things like that.
Actually DB, Automator & SL all seem to do just that, accessing your info in yet another way, so you'll be able to do what you want in more & more places and ways.
Also Tiger's iSync will support my cellphone, finally.
Just a thought.
Some major changes between updates are said to rely on things that are in the core of the OS.
Like, no, this or that iLife app will not work in panther because it relies on the new coreaudio or whatever.
It seems to me like this is becoming a way to sell software more and more, to make updated apps depending on some core functionality in the OS, so you'll have to upgrade completely.
Maybe there's a programmatic reason to it, but I think they also use it as a marketing approach.
Anyway....