My users love their little "karma wars" with Smites and Exalts. The like button doesn't really do this because there is nowhere to mark posts you disagree with or "smite". I really don't want to import that SMF board until karma is available.
I suppose then a reputation system could be a core feature, but in this case making the language easily adjustable. So that if you don't want to use "Reputation" you can call it "Karma" or "Health Points". Never know.