Terrain, Snowfall, off hill activities, nightlife, access
Park City is the best combination of everything. It has great challenging terrain, great snowfall, and a great town that offers a ton of stuff to do once you are done skiing for the day. However, that perfect combination tends to lure a lot of people.
The resort is right off the interstate, and less than an hour away from the SLC airport, so you get people from all over the country just packing this places slopes like its' nobody's business. Fortunately however, most of them tend to be midwest/Texas types who tote their families along, so the steep and deep is generally ripe for the picking, especially if you are willing to do a little sidecountry hiking or traversing.
This place really does cater to everyone, even more than Deer Valley, because it actually does allow snowboarders, and it actually has a quite decent terrain park that will keep park rats busy all day. The price isn't the best, but the variety of other activities it offers, including all the olympic stuff they built, really make this place one of the best to go to if you want to do anything other than ski.