This has been kinda dormant for a while, but I've been playing through a variety of games and thinking about what they don't have.
When you choose a class in Final Battle Adventure Online, it shows you what percentage of players have chosen that class. It tells you:
15% of the players online currently are Clerics.
18% of all players are Clerics.
It's not perfect, but it gives you an idea what to expect.
I've found games where a majority of the late-game content requires a six-person party, which requires a specialized tank and healer. If you found out that 40% of the players are tanks and 12% are healers, you'd probably choose to be a healer so you could get into those parties without spending an hour waiting for a healer to come on.
We'd also mention things like: "this class works best in a group" or "this class is recommended for advanced players" because the Social classes, for instance, only really work if you've got a regular group going.