According to Apple Insider, OS X's upcoming 1.06 update, Snow Leopard, will at least have one immediate benefit for all upgraders: it'll shrink the bloated baseline of your OS X install significantly.
The big shrink probably comes mostly from Jobs giving PowerPC support the hobnailed boot in 10.6. Without having to key relics of IBM bloat around, Mail.app will drop from 287MB to 91MB, iChat from 111MB to 52MB, and iCal from 89MB to 48MB. Even the tinier apps will shrink: Calculator will only be 2MB. At the end of the day, the applications folder will be a mere 25% the size it was in Leopard.
Apple, my gasping hard drive thanks you. It's already oozing bytes from both ends.
Update: Check out the comments. There's some fascinating stuff in there from people telling me I'm wrong, and what the real bloat from OS X comes from. Thanks, guys!
Five Undisclosed Features of Apple's Mac OS X Snow Leopard [Apple Insider via Gizmodo]