For us, the unit size (number of bedrooms, square footage, kitchen size, etc.) is the biggest factor. We had a bad experience being cramped in a very small 2BR unit on one of our first timeshare stays (as contrasted with a 2000+ sqft 2BR unit with vaulted ceilings in Hawaii for our very first timeshare stay), we're a family of 5, and we're used to a large house.
A distant second would probably be the quality of the resort (or at least the unit we'll be in). We could care less about ornate decorations, an elaborate lobby, or things like that, but good soundproofing, comfortable beds, well equiped kitchens, jetted tubs, grills and a nice pool are very desirable features for us.
Third would be our effective price for the exchange, including the exchange fees, the maintenance fees for whatever we used to exchange, parking and other fees. We don't like to pay more than $200-300 per week for a low-end resort, $400-500 for a fairly nice average one, or $700-1000 for a one of a kind resort (DVC, Harborside at Atlantis, NYC, or top Hawaii resorts). (And we're often able to do much better than that.)
Ratings and reviews (both from RCI and TUG) help us determine the sizes and features and any hidden costs, and the general ratings will be a tiebreaker. We like to get the best possible resort for the price and features.
Location usually doesn't matter much to us. We're usually pretty mobile. On any Hawaiian island, we'll drive all over the island and seldom visit the same places more than a couple times per trip. Beachfront would be nice, but we don't really care. If we have something specific (like an upcoming tip to Vegas for a conference, Disney trips, NYC), location will be more important.
