the additions of spotlight, expose, and dashboard were huge.
Expose was the only addition I'd call relatively big. Searchig features have always been there, spotlight itself is just an advancement of what is already there (and arguably should have been in since 10.0) and dashboard might be huge to a gadget freak, but to anyone else there's been things to do the same job(s) for years. Not huge.
Kind of like how Vista adds the spotlight clone, expose clone, and dashboard clone...
Actually, MS envisioned/described what would have become WinFS before Spotlight even existed (and WinFS still hasn't made it into Vista), there is no expose equivalent period and yeh... gadgets. I turn them off personally.
Vista is a mark above XP for the new gfx driver system and the new audio subsystem. Nobody's pretending it's a major step up from XP, apart from MS and everyone who can't see behind Glass.
here's Apple's list of the 200ish new features that were added in the upgrade from 10.3 to 10.4
Glancing very quickly at the list, it appears to be on par with an XP service pack to me.
The only big OS improvements that aren't actually some bundled software are Core Image/Audio and Aqua's 3D accelerations, which again, should have been solid since 10.0. Those features alone actually make Mac OS X -usable-.
As for the differences between the other versions, ... I'd suspect that there were similar levels of improvement between them.
I read in depth articles written by <insert memory loss here> of each release and the differences between that and the previous one. Each article was between 5-20 pages and reading them during work, it took me some days to finish. Suffice to say, I was well versed in OS X evolution about 1.5 years ago, but having not spent any time with them since I have plum forgotten everything. I did however come away with the knowledge that in fact no, they were not big deals. The first few versions simply made the OS almost usable, 10.4 is what the OS should have been from the start (arguably fully usable).
I know for a fact that there was a GUI change between 10.2 and 10.3,
There were GUI changes between every version I believe, although 10.0 to 10.1 may not have been significant (from metal to some other shinier metal to jet black basically)
not as drastic as the change between XP and Vista (more like between XP and 2003 server),
XP and 2003 have the same GUI. You may have meant 2000 to XP, although it's actually more comparable to changing from one XP Theme to another. It's still all the same stuff, it just looks different. Eye candy alone does not a good OS upgrade make.
the bundled apps (like Mail.app) have been constantly improving.
One should hope so. They were a disgrace in the beginning.