Terrain is key.
Snow amount/quality is second. The truly great mountains are the ones with excellent terrain and enough snow to actually make it skiable. Terrain is only as good as the snow on top of it.
Acreage is pretty overrated in my eyes. I would much rather ski a smaller area with less people than a mega resort- powder days have a distinct tendency to last longer.
Vertical is nice, but as a poster above said, most people aren't skiing more than 1000-1500 vert at a time, and places where expert terrain can be skied summit to base is exceptionally rare. Advertising 3000-4000 feet of vert usually means 1000 feet of interesting stuff and 2000-3000 feet of groomer to get back down at the end of the day.
After, terrain, snow, and vert, I think the most important thing for me is ease of access- not just getting there, but being able to park less than miles away (for free) and the like. Resorts that act put out to actually provide you with the facilities to throw money at them really piss me off. This includes space for parking either close in or with bus service that drops you close to the lifts, a place to stow a bag, etc.