Waaaay back in the early days of Stack Overflow Jeff wrote a blog post about the Stack Overflow Question Lifecycle.

Who decides what questions don’t fit? Trusted members of the Stack Overflow community decide which questions belong on Stack Overflow. Every question goes through a community vetting process:

1. You see a question that is inappropriate for Stack Overflow because it’s not programming related.
2. You have 3,000 reputation, the minimum required to cast close or open votes.
3. You vote to close this question.
4. Four (4) other users also vote to close this question, reaching a total of five (5) closure votes (or, a moderator votes to close — moderator votes are binding.)
5. Once closed, the question can be reopened by voting to open in the same manner. If the question garners five (5) votes to reopen, the process starts over at step #3.
6. If question remains closed for 48 full hours, it is now eligible for deletion.
7. You have 10,000 reputation, the minimum required to cast deletion votes.
8. If the question gets three (3) deletion votes (or a moderator vote), the question is deleted.
9. Note that deleted questions are invisible to typical users — but can be seen by moderators and users with 10,000+ reputation.
10. The question can be undeleted at any time by voting for undeletion in the same manner. If the question garners three votes for undeletion, the process starts over at step #7.

In my experience closed questions hardly ever get voted to delete afterwards. According to this Data.SE query:

• Exact duplicate 2994x
• Off topic 2444x
• Not a real question 640x
• Subjective and argumentative 481x
• Too localized 320x

So if we exclude the duplicates, which we tend to keep around, that leaves us with 3885 questions that are eligible for deletion. That's still 4% of all questions, which I think is quite a lot.

Because we don't keep negative comments, spamming users and off-topic answers around, I don't see why closed questions should be any different. So next time you come across a closed question that you think has no worth staying around (which is nearly all of them), please vote to delete them! That way your fellow users can help clean them up. If you don't have enough rep, but still want to help out you can flag them for moderator attention. You can also post them in the Super User Vote to Close/Delete chat room, where users with enough rep to close and delete can work together to get enough votes to take action.