Feature suggestion: an alert for a post that you've been tagged in (alert already happens) but also when that post is subsequently edited, since the edit could be significant. Quite a few times, I've looked back over a thread and noticed a material edit to the post and sometimes I have to make them too. This should probably only happen after the initial 5 minutes a post has gone up, though.
We could go mad and say that edits for any posts in a thread that you're subbed in to send an alert, but I think that would likely be too many alerts.