An "edit" function
This has come up in the past: the ability to edit a post once it's up to add things or fix a mistake.
Assuming it's even feasible, the down side, of course, is the large amount of "laundering" that people would be tempted to do.
I have posted to some blogs where you post, then get a timed "window of opportunity" (about fifteen to thirty minutes) to make any changes, then they freeze. You can fix mistakes, but you can't cover your tracks later. This seems to be the best of all possible worlds.