This question already has an answer here:
Have you ever gotten an answer soooo good, you wished you could give a +20 instead of +1 ?
That's a good mechanism, but they're not always right for this situation:
Bounties are usually associated with giving incentive to answer or draw traffic. I would expect that most users offered bounties before getting an answer, not after.
Bounties are usually offered by people with plenty of reputation. "New" users should be able to reward exceptional answers too.
Thanking in a comment
A warm comment on the answer is always nice, but I'm not sure comments are intended for saying thanks - but to clarify / improve the answer itself.
So what is the recommended way to reward this truly exceptional answer?
I know there is an answer of the week blog, is there a way to nominate an answer to be there? Sometimes exceptional answers are hidden in low-interest questions that receive less traffic - so those will never be upvoted enough to become noticed.
Or maybe instead of a single blog for the entirety of SO, there should be an entire section called "Answer of the week" with more than one answer (maybe on a per-tag basis). Answers should arrive there based on manual nominations and not on upvotes so unpopular questions will not be discriminated against.
It's not that I'm personally against bounties - I use them plenty of times. I'm mostly concerned about what other people tend to do. I suspect the bounty post-answer reward is not used as much as it should. I believe the majority of people don't consider bounties for this purpose.