I unconfidently think it would be good if there was some way to promote posts on LW (and maybe the EA forum) by paying either money or karma. It seems reasonable to require such promotion to require moderator approval.
Probably, money is better.
The idea is that this is both a costly signal and a useful trade.
The idea here is similar to strong upvotes, but:
It allows for one user to add more of a signal boost than just a strong upvote.
It doesn't directly count for the normal vote total, just for visibility.
It isn't anonymous.
Users can easily configure how this appears on their front page (while I don't think you can disable the strong upvote component of karma for weighting?).
That's not really how the system on Stackoverflow works. You can give a bounty to any answer not just the one you accepted.
It's also not lost but:
If you do not award your bounty within 7 days (plus the grace period), the highest voted answer created after the bounty started with a minimum score of 2 will be awarded half the bounty amount (or the full amount, if the answer is also accepted). If two or more eligible answers have the same score (their scores are tied), the oldest answer is chosen. If there's no answer meeting those criteria, no bounty is awarded to anyone.