A bit of history...
Over a year ago, when we were still in beta, a lot of users had 4k+ reputation (which is the equivalent of 20k+ in beta) and closed questions used to be removed by 3 community votes very regularly. In fact, so regularly that multiple users would exhaust their delete votes on a daily basis.
This slowed down once the site graduated, because a lot of users suddenly lost their privileges or remained in the 4k–20k limbo for a very long time (16k is a long stretch, and the reputation snapshot on D-day shows that we only had about 5 20k+ users for a while, of which 1 was already a mod and I later joined the team). Over long periods of not having the ability to delete immediately (10k users need to wait 2 days), people simply forgot that deletion of closed questions is also a part of community moderation. Very rarely do users remember to go back and delete a closed question after 2 days.
I don't think that the problem is that people disagree that closed questions should be deleted (of course, there will be minor differences of opinion on what should be deleted), but rather that new 10k/20k users are not aware that deletion is part of the process and that the others have simply forgotten about it, since there isn't an easy way to get to a list of questions that need delete votes. There have been talks of introducing a "delete queue" in /review, just like the close and reopen queues, but it hasn't been implemented yet.
Here's some data from the moderator tools that supports my claim:
Up until Aug 2012 or so, questions were being deleted at roughly the same rate as they were being closed. From Nov 2012 onwards, the deletions started slowing down and now (Aug 2013) there's a significant gap between the number of questions being closed and those being deleted. This widening gap is a consequence of us growing in popularity (more crap) and fewer users (than in beta) having the ability to delete immediately. In fact, a lot of the question deletion activity in 2013 is either moderator initiated or automatic (by Community ♦). Only one 20k user assists us in deleting questions on a fairly regular basis.
One of the reasons for question closures being more regular is that the revamped queue system has made it very easy for users to review pending closures and skip if they're unsure. Perhaps a delete queue (whenever that is introduced) will have the same effect on community deletions.
(If someone's curious: Answer deletions were higher during the beta because there was a LOT of activity relative to the number of questions/day and several users would post more or less the same answer simultaneously. Most of these were self deletions.)
My view on which closed questions should be deleted:
I think that most closed (and by closed, I mean non-duplicates) questions that are not edited and improved, should be deleted eventually. The system provides a lot of users with the tools necessary to prevent deletion, if there's some redeeming value
- Anyone can suggest an edit to improve a closed question
- Users with 2k+ can edit a question to improve it
- Users with 3k+ can, in addition to the above, vote to reopen it.
If after a sufficient time (I personally view 2 weeks as sufficient time), none of these have happened, then it probably should be deleted.
However, people hesitate to delete some of them, because someone has answered it and they feel guilty of "robbing" them of reputation. This is one of the reasons why I generally don't encourage answering terrible questions that will eventually get closed, because it confuses those wanting to delete. As Mr.Wizard made it clear in his answer to the linked question, if someone has answered a poor question in the interest of helping the user, then they shouldn't have any expectations that the question will be around forever. Also, now that users can see their own deleted questions (and the answers on it) regardless of reputation, there is no reason to keep them around so that they can see it.
Also, if poor questions are simply closed with no answers or no upvoted answers, then the Community ♦ user will automatically clean it up every 2 weeks/month, saving 20k users the trouble of doing it themselves (they can spend their votes on other stuff). However, posting pity answers, or worse, upvoting such pity answers disrupts the roomba, causing them to pile up. Right now, I really can't think of any closed question that deserves to be closed, but kept around on the site. There probably aren't many like that...
If anyone's looking for a list of closed and downvoted posts, they can use this search (we don't have an easy way to filter out duplicates yet).