I'm throwing this in the Advanced subforum since it's just me rambling instead of just a request to Andrew.
The one plugin I would like to see is an email to confirm a comment. Some of you may have noticed that if you leave a comment on my blog, I'll email you with my response before I post it. I do that simply to confirm the person behind the post actually exists and isn't a bot. (I also do it to be polite to be honest.) If the email bounces, chances are it's a bot or at the very least a faked domain.
Sure, spammers can get around this but just using drop boxes. They do it with splog creation scripts. But at the very least, it's yet another method to prevent spammers.
And, no, MT doesn't have it either.