The thing with Vail and difficulty is that it is just difficult enough to challenge most of the crowd that skis there, yet tame enough not to scare them off. Challenging terrain exists there, but one really has to go searching for it. There are a few steep trails, but again, you have to know where to find them. Skiing at Vail is not like going to Jackson, Snowbird, A-Basin, Alta, or Whistler where at any time on any run (just about) you can find terrain tough enough to make you need to change your pants - stuff that if you fell on it you would likely die or be seriously injured.
At Vail if you're looking for some fun there are cornices in the back bowls, fresh snow almost always off Lover's Leap and Iron Mask (as well as a few steep turns in there as well), Hollywood cliff on the front side, the rock gully in Blue Sky, as well as some cliffs to the right of the main chair (going up), and a few semi-decent steeps/drops in outer mongolia bowl (I think?). Plus there are also the front-side chutes that are in the Aspen forest (Been There, Done That, and a few others). They are hard to get to but usually moderately difficult skiing for a few turns.
All things considered though, there are not many places at Vail that are what I would call in-excessible to anyone who is at or above an intermediate skill level. That is a big part of the appeal. I know of people who deliberately do not take vacations to places like Snowbird because they are too difficult to ski at (little grooming, narrow groomer runs, cliffs, rocks, chutes, trees, etc.). Vail is like a big play ground where you can release just about any level of skier out onto and they will have a good time. True experts probably won't be challenged - but if you are really a true expert you probably have to hike, heli, or take a snowcat in order to get really 'hairy' terrain anyway. Luckily, skiing Vail isn't about the challenge... if skiing Vail is challenging for you, then you probably aren't a big challenge seeker anyway, and Vail would probably be more fun for you than going to a mountain like A-basin or Alta.
Like I said above, Vail is more about experience than thrill seeking, adrenaline, and extreme terrain. If that is what you seek when you ski, then Vail is probably not the place for you... but I assure those who have never been there, that few other ski areas can offer the feeling you get when you are the first to the top of the back bowls on a sunny, blue sky powder day. If you have been there under those conditions - you know what I am talking about.