Pluses at Keystone:
Helpful, well organised (this is N America, not Italy after all).
Because of the recession and the time of year I went (late March), it was fairly quiet and no/tiny lift queues.
Enough skiing unless you are really hard core. You can always go the the nearby resorts of Vail, Beaver Creek, Breckenbridge if you want (same lift pass but need to take bus or go by car).
Lots of accommodation nearby - little that is ski-in, ski-out, but little problem unless you are lazy or can't walk & hold skis at the same time. Superb free bus chuttles that take you all around Keystone.
Well-organised night skiing.
Fantastic ski school (I used Mountain Lodge and they were the best I have used - good for all levels)

Minuses at Keystone:
The elevation can take getting used to for some.
The cost of the lift pass - ouch. Made worse for us Europeans by the terrible exchange rate. However, a season pass for all the local resorts costs roughly the same as a week pass so actually it is cheap if you go for a few weeks.

Barry Peters