What's the most important thing to look for when buying used?

For me, it's mileage above all else. If your ad doesn't list mileage, I'm skipping it. Mileage is the chief determinant factor of value in my opinion. One of the top reasons I bought my Jetta was because it had only 23k miles on it. But some people don't worry about mileage, which blows my mind. So what do you think is the most important criteria to consider when shopping for a used car?