In some sites, Stack Overflow being the most significant, a question without an accepted answer after a few minutes has a very high probability of never being answered.
Opening a bounty could minimize this problem (for users that can "pay" it), but it means a delay of 2 days. See here for a very good discussion on this interval. It seems that site time is too fast, compared with clock time.
Must the reward of an accepted answer be proportional to the time between question and accepted answer?
Another possibility (just an undefined idea) could be to use "interesting tags" as a way to "split" the audience in very big sites as in smaller ones.