Warcraft is probably a bad example. While DKs are overpowered due to some abilities that are obnoxious in PvP, those same abilities are of limited use in PvE, and DKs will never serve in any healing role, and offer limited part-wide buffs.
As someone who does play the jack-of-all-trades druid class, my versatility comes at a cost. I can tank or heal, but not at the same time. If I tank, my heals and mana pool are about %50 of what they need to be. If I heal, my health pool and damage are about %50 they need to be. Playing both roles means that I have to grind and play twice as much for the same gear. The game forces characters to specialize if they are going to be effective.
Perhaps a better example of an open-ended character development model is EVE. But there the limitation on character development center on time and money. If you've invested the months of training time required to get into elite mining vessels, you are looking at more months of training time to get into capital warships. The estimated time required to master all skills for all roles in the game is considerably longer than the game has been live.
At any rate, it is a basic of game design to force characters to make hard choices regarding what kind of strategy they will develop. This is true for MMOs, and it's true of older games as well.
