voloch: I do not think a user can delete each of his/her questions. I was under this impression, too, but in some discussion it came up that there is a certain threshold of activity, regarding answers and alike. If a question is past that threshold, a user cannot delete it anymore. Also I have some vague memeory of a dicussion on SE to the extent that expansive deletions are or at least can be considered as site-vandalism. Finally, there is also the thread 'deleted question with (possibly substantiave) answers' on this board whose sole purpose is to restore deleted questions. Thus, the situation seems not so obvious.

Update on the 'threshold': From the linked thread according to Anton a question having an answer with at least two upvotes cannot be deleted by its owner.

@Joel: what do you mean by 'I have the options' that you did/tryed this or that the link are there. (Trying it might be risky for any <10k user as one would be unable to get back to the question if I understand correctly.).

I do not want to insist that my information is correct, as I never tried it, but what I know is that I have a link to vote to close a question even if I did so already, and only when I click do I get the message that I cannot vote a second time. So, just that a link 'delete' is there might not mean much.

Added: Of course I cannot know whether User's questions are above the threshold, so it might be the case that s/he actually can delete, but did not know how. Though in view of the very first sentence of the conversation this seems surprising. But who knows.

Update: For the puprose of testing and since this could be restored by myself, I just tried to delete one of my answers which is an accepted answer. Delete link was there, I clicked got a pop-up to the extent 'really delete', I clicked yes and only then was I told I cannot delete as it is an accepted answer. So, I am quite convinced that the information I gave is correct.

Your own questions and answers are always be visible to you, even when they are deleted. However, for some strange reason, deleted questions and answers will not be visible to you on your profile page until you reach 10k. You can work around this by noting the url (or just the question number) and typing it in directly to reach your deleted question or answer. If you completely lose a deleted question, ask a moderator to find it for you.

You have the ability to delete/undelete your questions as often as you want. However, if your question already has answers with a non-trivial amount of votes, then you will need community approval before your question is deleted since that will also delete other people's answers. In those cases, flag for moderator attention or email the moderators directly.

It's perhaps not well-known that you can always vote to close & reopen your own questions. If your question gets deleted and you vote to reopen it, then your question shows up in a list for 10k users, who may see it and decide to vote to reopen too. If you don't vote to reopen your question, you lose that extra visibility.

I was on the University of Michigan graduate admissions committee this year. I googled candidates whom I thought were borderline or unusual, and several of my searches found MO accounts. In my opinion, the impact of these were always either positive or neutral. There were several undergrads where I made notes in their files like "Clear writer on MO" or "Asked some interesting questions on MO about Lie groups". If the account showed low level questions, who cares? We are looking at undergrads; the fact that they are asking questions at an undergrad level doesn't reflect badly on them, if anything, it reflects mildly well that they are looking into professional fora.

The exception (which didn't come up) would have been if someone were frequently rude on MO. Our committee chair asked us to flag any files where we got clues that the applicant might have a difficult personality. I didn't see such clues at MO, but I would have noted them if I did.