Powder! Easy access. Variety.
Visibility, wait for the gondola, avalanche warnings
We went in early April, and got lucky - 30" of snow in the few days we were there. However, our skinny, east-coast-centric skis are terrible. If I go again, I'll at the very least rent some fat powder skis.
It's easy to get there - fly into SLC, and there are plenty of services to drive you there. You can fly from pretty much anywhere in the US and be skiing the same day.
The terrain park was pretty much non-existent. Like one or two rails or something. If you look at the trail map, you'll see this tiny area dedicated to it. It's far less than what the map makes you think it is. That's not detrimental to me - I'm not there for that stuff.
The lift lines were always short. I don't think I ever waited more than 2-3 minutes, if that.
The nightlife is about as non-existent as the terrain park. But after a full day of skiing, I didn't care.
The back bowl was closed most of the time we were there - avalanche and visibility problems. But when it did open up, it was great. FEET of untracked powder. At one point I fell - when trying to get up, my pole sank all the way in (48"), and never hit anything solid.
With all the snow that came, the visibility was terrible. The few times it let up was a blessing; some of the trails can get very narrow, and you're skiing based on intuition and luck.
There are a variety of restaurants on site, that are pretty good. The prices weren't too bad at most of them, either (especially considering the usual resort mark
The resort might be boring outside of skiing, but that doesn't matter to me. I'm there for one thing, and Snowbird does it perfect.