The PlayOn project at Xeroc PARC has been analyzing WoW data and then posting it on their blog. (These people get to play games at work!) Thanks to richardt for the link. Some findings I found interesting:
They computed the fraction of time each class joins a group. As you might expect, Priest was in a group much more often than any other class. Priests also level up faster than other classes. But that seems to be an anomaly. In general, people in groups level more slowly.
People are more likely to be at a level that's a multiple of 10, probably because there are certain rewards at levels that are multiples of 10, or there are penalties (such not being able to play on lower-level Battlegrounds). So people at a level just before those levels will play extra hard and people at those levels will try to avoid leveling. Leveling time in general is roughly proportional to the level, so you get a quadratic effect. There's a spike at level 40, most likely because that's when you can buy a mount.
Warriors are the most popular class at low levels. Hunters are the most popular class at high levels. Shamans are the least popular class. Humans are the most popular race. Orcs are the least popular race, despite them having the best dance moves.
It's a fun blog to read.