I saw a funny but informative dev talk on classes in games, where a guy was talking about how most popular mmos had like 1-2 tanks, 1-2 healers and then a gazillion dps classes.
He went on to explain the fun factor in each, and how it correlated with the type of people likely to play mmos, and ended up concluding that making a new tank or healer was in most cases a wasted effort because it'd attract only a fraction of the people a dps class would.
Pretty interesting.