Long runs, tons of groomers, Crowds spread out well
Expensive, lack of terrain variety
If you love cruising well-manicured blue runs, like myself, then you will thoroughly enjoy Keystone. If you are an expert and don't mind working a little to get to your terrain, then you will find a bounty of high-alpine, open bowl skiing. Keystone also offers cat skiing to some of these bowls for an additional cost. If you are a terrain park junkie, you can't do better than Keystone's A-51 zone. This area truly can be called a "park" as it is huge and has something for everyone, from jumps over 50-foot gaps to little dip runs for those of us just wanting to get off the ground to rails and boxes.
Keystone's strength is the length and quality of its runs, especially on the front mountain, where you can take 3000 vertical feet on every run if you'd like by making laps on the river run gondola or summit express quad. I would advise using the summit express quad if conditions are not extreme. It will always have a shorter line (usually non-existent), and it does not stop at a mid-station like the gondola.
I have mixed emotions about these runs on the frontside of Dercum Mountain (the first of 3 mountains at Keystone). I already mentioned how long they are, and another positive is that they are constantly changing pitch. There is no one consistent fall line, and they meander quite a bit, so the runs have variety to them. However, this is a very mellow mountain all the way across, and while each individual run may be interesting, they are all pretty much the same. For instance, Paymaster, which is blue, is no steeper or narrower than Silver Spoon, the green right next to it. Basically, if you ski all of the runs on the front side of this mountain, you will feel like you've repeated the same run over and over. The exception is Lower Go Devil, which is a black that was groomed the last time I was there. It was incredible, with perfect conditions for smooth, fast turns.
Probably my favorite run at Keystone is Mozart, which is the blue run off the back of Dercum Mountain that takes you to everything else the resort has the offer. Because of this fact, it is very busy, but it's huge (100 yards wide in places) and can handle the traffic. The top part is comparable to Mary Jane at Winter Park, but the bottom gets tight and steep before ending in a long runout. If groomed, Diamondback (a black route to the backside) is another excellent choice and can be handled by an intermediate skiier.
I personally don't feel that the blue runs on North Peak or the Outback are all that interesting. Oh Bob is probably the best, but the others at the Outback are just cut straight down the mountain, unlike the runs on Dercum, making them fairly boring and consistently pitched the whole way. There are no real surprises here, but it's still worth going back there to check it all out, because you feel like you are miles from civilization.
If you are not skiing blacks, then on your way back to the front you will either take Last Alamo or Prospector. They are very similar, and each one bears the brunt of any breeze. These can be absolutely miserable on a windy day, so you might avoid going back here if the wind is howling (or opt to take the Outpost Gondola back to the front).
While I would definitely ski Keystone again (I've been there multiple times already), it certainly is not my favorite mountain. I like it better than Breckenridge or Copper Mountain, but it can't touch Vail or Winter Park. One thing I would note is that the size they advertise is deceiving. They say they have 3148 acres of terrain, but half of those acres are in the black bowls that have no lift access. For non-experts, the skiiable terrain is more like 1500 acres. For families, it's less than 1000, because only the front of Dercum Mountain has beginner terrain. Don't misunderstand me, though. There is plenty of fun to be had on those front runs.