We originally rolled out the question bounty system in January 2009. It has worked fairly well, in terms of motivating answerers and increasing the connection between askers and answerers. But I think we made some mistakes in being overly strict in our design of the bounty system.
Just take a look at some of these highly voted meta requests in the [bounty] tag:
How about making it possible to set up bounty on someone else’s question?
Why an answer can’t be accepted after an unresolved bounty?
Please do not auto select answers on bounty questions
Why do I always have to accept an answer at the end of the bounty period?
Should SO introduce a Re-Bounty feature …