A preview option is great for proofreading your post because as anyone who has tried to re-read something they just typed in these reply boxes knows, your brain knows what you intended to say and will fool your eyes into missing a lot of errors that are very noticeable once you've hit the button of no return.
An edit feature in the main comments section would be the bee's meow not only for sparing oneself from embarrassment, but it would cut down on posts from people who come back to correct trivial mistakes from their previous posts. ("In my last post I didn't mean to say DoubleGames SOCKS...")
And maybe, just maybe, when certain folks saw how childishly they were behaving (or had it pointed out to them) they just might moderate themselves.
In the event of abuse, a limit on length could be considered. I think Jucati has come up with an interesting suggestion.
Incidentally, I propose that overly verbose, tedious wind-bags who drone on and on and on like, oh, say me, for instance, but are otherwise harmless, should be exempt from the length limit.