"... Here's a random idea... Display a text box that requires a reason for flagging the post or comment. ..."
It's not a bad idea but it has the same problem as trying to describe why you like something instead of agreeing. I know there's a talk by Barry Schwartz on decision theory ~ http://www.google.com/search?q=paradox+of+choice that argues against explaining instead of just choosing. A flag is enough. People around here are pretty smart. The reasons why will mostly be obvious.
The only thing I don't like is it ius close to the "delete","parent" tag. My clumsy mouse-hands might hit it by mistake.
Isn't that premature?
And if some really use it inappropriately then they could just lose their right to flag. No need to bother the vast majority who will use it when it's deserved.
Whether it's premature or not, I'm just putting the idea out there. The question is, what does that take away? If the item is truly flagworthy, then it shouldn't be any problem filling in a box.
The added benefit is that you have much less accidental flagging.
The question is, what does that take away?
Simplicity.
That would cut down on flagging for the sake of flagging.