I really like Windows (Vista now) because it got the best ui concept. It's great to work in full screen mode and be able to see and reach all tasks over the task bar. Windows has the right features that I need (especially Media Center, Games, powerfull
file management capabilities) and it works great.
Before I would switch to Mac OS, Apple should come up with a better solution for managing windows and apps. You can feel that the old Lisa kind of concept, the NextStep concepts like the Dock and the new features on top (like Exposé) didn't really match together.
Furthermore the Mac OS should support a better font rendering engine (it's sometimes hard to read text really fast), should support current Games and should have a powerfull integrated Media Center and Tablet PC functionality.
But maybe the biggest barrier for me to switch is the smugness of Apple. You can't really be critical, Apple users are kind of religious.