One question you can ask at the conclusion of the interview that will not necessarily put them on the defensive, and one rarely asked, is:
"Do you have concerns about any of my qualifications for this position, and if so, may I have the opportunity to address those areas further with you?"
This does two things: 1) demonstrates you're interested in improving by being open to feedback; 2) gives you potential weaknesses you can work on for future interviews.
I'm in job search mode right now, too, and I've gone on so many interviews I'm starting to feel like a "professional interviewee". At the end of the day, I would recommend you ultimately don't give up and improve in whatever areas you can.
Ultimately, you don't often select the right job, it selects you. Best wishes with your search.
[ March 28, 2008: Message edited by: Jon Guenther ]