Caveat: we love Waterville Valley -- so much that we have recently moved full-time to the town. That said, I think I have a pretty clear view of what the area does well and not as well.
As others have noted, the terrain is biased toward the blues and navy-blues, with several more-challenging runs over on Sunnyside. Depending on conditions, True Grit (recently cited as a "Classic ski trail" in Ski magazine) and Lower Bobby's can test very good skiers indeed. The balance of the terrain -- especially Tippecanoe and Tyler Too on the north side -- provides solid intermediate challenge and high-speed cruising on broad slopes. There are several older, New England-style trails off the summit as well. Interestingly enough, there is relatively little novice-friendly terrain from the summit.
Down lower on the mountain, Valley Run (served by the Quadzilla chair) is a great broad teaching slope with consistent, gentle pitch. Stillness is a sweet easy intermediate run off the same chair. Lower still, the Lower Meadows teaching area provides gentle learning terrain off an easy double chair, and the KinderPark J-bar and magic carpet are great for newbies.
At 2000 vertical feet, Waterville Valley is not a small mountain, but the variety of chairs mean you can pile up a lot of vertical feet in a day. The area's strong race program (BBTS and Waterville Valley Academy) mean there are lot of very good skiers on the mountain on any given day. There are frequent races, too.
I have seen a renewed and vigorous commitment to snow making and grooming this year, and, in spite of the poor weather in December and January, Waterville has had consistently good conditions and as much or more terrain than any other New Hampshire ski area. Waterville is also 1000 feet higher than sister resort Loon, and often has better natural snow conditions as a consequence.
Waterville has several terrain parks and has expanded their "freestyle" terrain all over the mountain, including small, medium, and large mogul fields. They added a Burton Progression park for the 2007 season. Waterville usually tries to open two halfpipes - a smaller one in Little Slammer park and a large one in the Exhibition terrain park. I am not a "park guy" -- but all the facilities seem very popular with people who are.
Services at Waterville are generally good, though the facilities are mostly older. The main base lodge can get very crowded on weekends, but on-mountain restaurants at Sunnyside Timberlodge and Schwendi Hut (at 3800 feet) are both nice alternatives for lunch or a break. Food on the mountain is about average in quality and price, though the fondues and other specialties at Schwendi Hut are pretty good indeed. Waterville did upgrade several bathrooms this season, which were sorely in need of attention.
Waterville Valley really shines in all aspects of family services. From the Kids' Kamp program (day ski programs for kids 3+) to the excellent Seasonal program (all-day weekend program for the full season) to the BBTS racing programs, WV cranks out lots of good skiers -- and lots of happy families. The adult ski-school is also top notch.
Waterville is situated in the White Mountain National Forest, and surrounded by 4000 foot peaks. The scenery is tremendous, but you aren't going to find the wild-and-crazy apres scene you run into at the big Vermont areas. There are several cozy bars on the mountain, and several nice restaurants in the town proper -- but you don't go to Waterville if you're as interested in howling at the moon as you are in skiing. That works fine for me -- I am here to ski.