I've never been a member of the "one of everything" crew. My typical realm loadout is a level-capped priest, a mid-range paladin and mage, sometimes a high-level rogue, and a banker -- and then more priests, paused at varying states of leveling up alongside now-inactive partners or guild groups. I've earned my understanding of other classes through hours painstakingly assembling raid strategies during early WoW (before the advent of the Dungeon Journal and boss videos) or, more recently, via hard knocks in PvP -- the best teacher, in my humble opinion. Nothing lends a dawning awareness of how another player's skill works better than an injection of quick, brutal death with it.
How familiar are you with the mechanics of today's WoW classes? Do you know all the contemporary systems like the back of your hand, or do certain specs or classes remain a mystery? Do you play the classes you seek to understand, or do you conduct your field research in other ways?