Well, Tiger wins the prize as the most brain-dead-simple, non-problematic O/S upgrade I've done. I like Spotlight (more for the smart folders than for ad-hoc searching), hate (at least initially) the new UI for Mail.app and am ambivalent at this point about Dashboard. Since I haven't made use of Automator yet, all I can say is that I like the icon. Although I'm pretty sure that little robot guy is actually carrying a bazooka.

If you have Tiger, be sure to check out the Dashboard Widgets, Automator Actions and Spotlight Plugins, all hosted by Apple and of varying costs and utility.

Point of relief, by the way: WoW works fine on Tiger.