My two cents ... i agree with your point about clearing replys - but like you said, only if the author wants them cleared.

In terms of closing threads ... I personally myself would never want to close a thread ... the only time I could think of is when a thread gets so long (40 posts or more) that the conversation must be closed and restarted in a new thread ... at which point the moderator would post the message saying this thread is closed - here's a link to the new one.

-Shawn