Believe it or not, between all of us, I think we do read most of the posts. At least, I think I do! Sometimes we might get to a thread a little late, but eventually, they all get read. Sometimes we're alerted to a post by PMs from other members who may find something objectionable and we'll take a look.
Heated threads are not flagged in any way by the envelopes. The envelopes simply tell us when there's been new replies to a thread.