The ones that cause me the greatest heartburn are those that require a restart ofter installation. That usually means kernel extensions or other deeply embedded detritus. Happily CleanApp does a pretty good job of tracking that sort of thing if I have to remove something.
This is one more reason I have less and less software in my system that does not come through the App Store. That way I am assured Apple has vetted the software carefully and has a very strong interest in keeping the installs clean and the apps running within well defined boundaries and clean uninstalls are a piece of cake. It aint all beer and skittles however because Apple's restrictions imposes bounds on the developers and sometimes prevents them from doing something I would like, but at least it is one less thing for me to worry about.
