Only one? For me it's the cost of insuring the car; that's what kills you I think after having a thorough examination of it to see it is in good working order etc; the fact that I am a male and 25 automatically means I am paying more to be insured on account of the overly idiotic boy-racer types who try to impress girls with their cars. Otherwise, it would be running costs and looking for something economical and within my budget.
If you're looking to buy used, I'd make sure to have someone you know and trust check out the car first, in person and personally. A mechanic, a relative that's a guru when it comes to cars, or whatever. I got screwed, let me tell you. Car was great initially, they assured me nothing was wrong with it, but 2 weeks and less than 500 miles down the road I was left replacing an engine that they ended up charging me $3,800 for. Now I wish I would have had someone else look at it, 'cause it would have been avoided.
If they seem way too eager to get it off the lot, be wary.