It would be nice to be able to spend your credits rewarding those who go above and beyond answering questions, or to reward runners-up to the best answer. I've had some amazingly helpful people tell me the answer to my questions and at the same time tell me that the question itself had flawed suppositions. This ought to be rewarded somehow, I appreciate it greatly, as I'm sure we all do. It's precisely the point of StackExchange.
If you have at least 75 reputation points you can place a bounty on any question that was asked at least 48 hours ago.