Now if, back in 1978, you’d told me that there were going to be three main character classes in future MMOs, I would probably have assumed some kind of rock/paper/scissors relationship among them for reasons of balance. Archers beat infantry, cavalry beat archers, infantry beat cavalry — that sort of thing. I don’t believe for a moment I’d have gone with what we have, which is the “trinity” of tank, heals and dps. The tank takes all the damage issued by the opponent, the healer reduces this damage, and the dps gives damage (dps is “damage per second”, non-players) to the opponent. This doesn’t make a great deal of sense in gameplay terms: the healer is redundant (they’re basically just armour for the tank), the premiss is unrealistic (“I’ll hit the guy in the metal suit who isn’t hurting me, rather than the ones in the cloth robes who are burning my skin off”), it doesn’t work for player versus player combat (because players don’t go for the guy in the metal suit) and it doesn’t scale (a battle with 1,000 fighters on either side — how many tanks do you need?). Don’t get me wrong, it can be a lot of fun, but it’s a dead end in design terms.
Richard Bartle, “The Evolution of the Trinity”