Cost of living would be one of the biggest factors. I mean, low crime rates and decent weather and a great cultural scene are all important, too, but I see no point in moving to NYC or San Francisco and pouring all of my money — above the median national or city income or not —into housing.
Of course, I’ve never been a big city kinda gal in the first place, so population size is an important factor. I love visiting the big city, but I’d never want to move there. Your Columbuses or your Santa Barbaras would be about the biggest cities that I could handle.