Is it a bad idea to allow multiple ways to show approval for an item? Take a look at some of the following examples:
You can vote up or mark-as-the-answer to approve of a user's response. Why is voting up/down still enabled even after marking a response as the answer? If it's already the answer, how is it possible that I would disapprove of it by voting it down? Should I both vote up and mark it as the answer or just mark it as the answer? These are the questions that race through my mind when using this UI control. Don't get me wrong - I think Stackexchange benefits from both methods, but just not both on the same response.
You can both like a video and store it in your favorites to show approval for a video. If I store it as a favorite, I presumably must also like it. But Youtube still gives me an option to hit the like button after already favoriting it. When should I do both or just one? Why does Youtube even have 2 options? Favorites are useful for reminiscing about old videos that you've seen, but liking is useless for the viewer. I suppose liking helps Youtube's ranking algorithm, but wouldn't removing the like feature and relying solely on the favorite count suffice?