Pros: Size, diversity, location,
Cons: Cost, atmosphere
Killington is the most diverse eastern resort. It has legitimate steeps in the Canyons, world class bumps on Bear Mountain, including Outer Limits, and good intermediate and beginner terrain on Ram Head and throughout the mountains. It is big - so big that driving from the main lodge, K1 to the Skyship base area and around to Bear Mountain is about a 20 minute drive.
There are many things to like about Killington and some to dislike. When I first started skiing, Killington was really a great experience - you could go on green trails all over the seven mountains. There are some very long (like 7 miles) beginner trails (Juggernaut and Great Eastern) that are just a great skiing experience - through woods, over bridges, cross the road, etc. I still try to hit them at least once each year. There are some really fine steep trails in the Canyon that, given icy conditions, are a challenge for any level skier. Killington also has a real range of bumps going from ungroomed natural trails, through Superstar (usually half groomed) to the big Volkswagen size manufactured bumps on Outer Limits. Killington also has some excellent ski shops with knowledgeable staffs and good bargains if you hit them during a sale.
Killington also has an atmosphere that is not entirely pleasant. The staff can be friendly but are often just there. The mountain web site takes a happy face approach to conditions and snowfalls: it will almost always be less snow than they claim and, when conditions are not great, you wouldn't know it from the posted reports. Whiteface and Stowe have much more honest reports. Killington is also fairly expensive especially during early and late season when not so much is open. They have also been gradually doing away with the opportunities for savings and promotions. They tend to call every trail segment a trail to create this silly number of open trails (if they say 50 are open, there may only be 20 legitimate end to end trails.) They do a poor job controlling traffic at the numerous trail junctions and they get all levels of skiers and boarders so it can be a spot for some scary encounters and near misses as boarders pop out of the woods without looking and newbie skiers straightline down hills they can't handle. All that being said, I return every year to Killington and enjoy it.