1) Budget
2) Length of Stay
3)Theme
4) Whether we've stayed there before
I had a really difficult time deciding for our first on-property trip a few years ago. It was DH's first trip altogether, so there was a lot riding on it. If either one of us (mostly DH) didn't love the trip, we probably wouldn't go back very often, and wouldn't spend the money to stay on-property when we did go.
Now it's easier, because I know I'll likely get to make my rounds to all the resorts I want to try at some point, even if it's just a few nights for the more expensive options. I LOVE that there are so many options, because I can look at our budget and find somewhere suitable every time.
I imagine it will start getting difficult again once I've stayed at more places.