Didn't know which forum to put this, but since this is a question for hot dog buyers rather than vendors (although I assume vendors will be interested in the answers), I put it here. If I don't already know a good hot dog cart, these are the things I look for in choosing one:
1) Interesting condiments. In New York, a lot of vendors don't even have relish; it's mustard, onions, sauerkraut, and ketchup, and that's it.
2) Cleanliness. A vendor who pays attention to that will generally pay attention to the little details, as well.
3) Signage. I'd like to know how much I'm going to be paying for something before I buy it, and I don't want to have to ask.
4) Pride. If the vendor appears to be happy with what he's doing, and shows pride in his food (or, better yet, brags about how good it is), that makes up for a lot of sins.
How about the rest of you?