I just noticed that a question that was closed yesterday was responded to today, thereby bumping it to the top. Is that normal? I don't recall seeing this on SO, but I could be mistaken.
It certainly does happen on Stack Overflow, it's just that the activity level is so high there that they tend to get bumped down pretty quickly.
Closed questions don't disappear. The concept of a closed question is that it is probably going to be deleted at some point, but until then (at least 2 days), the author - or anyone else - can try to edit it into shape, or plead their case on meta.
Normally, you need to have 10k reputation to vote to delete questions. During the beta period, it's 2k. And speaking of which, I put in several votes to delete the other day but didn't get any help (they've since aged away). As the "sous-moderators" here, we all need to do our part with deletions.
FYI, the general thought on Stack Overflow (or at least MSO) is that we only delete Off-Topic/S&A/NARQ questions; duplicates are candidates for merges instead, which is something that needs to be done by diamond mods. We don't have any yet, but for now, I guess just leave them alone, and vote to delete any other closed questions after 2 days (unless you see a reason to keep them around).