You do lose SOME functionality that's launcher-related, like some hovering actions on the homescreen, Samsung Widgets will be inaccessible, some have reported not being able to use the Action Wheel if not on TouchWiz, but some others have been able to, so I'm not sure if all builds will be the same...
Chances are that if you don't like TouchWiz for whatever reason, then you're not really using these features that regularly, so probably you won't miss them and may not even notice they're gone anyway.