One particular problem is that when you point out a mistake in someone's answer, you have to check several times until he fixed it so you can delete your initial comment.
Otherwise your comment will likely be flagged as obsolete.
To avoid this issue, I was thinking about an option available when you post a comment, that would make the comment automatically auto-delete after one of four cases:
the poster edits is answer,
the poster acknowledge (with a little check box) your comment,
the poster respond to you,
a certain amount of time passes.
What do you think about such auto-deleting comments? What would be the downsides of this system?