Feature I use a lot that I didn't think I would: Quick Look
Feature I don't use as much as I thought I would: Spaces
Performance improvement that blew me away: Spotlight (it's amazing!)
UI changes that don't bother me: reflecting dock with glowing orbs and translucent menu bar
UI changes that do bother me: menu selection colour (too blue) and non-focused tabs in Safari (the combination of the embossing on the dark grey is horrible)
The original post was in the category: os_x but I'm still in the process of migrating categories over.