At 60 skill mattered first and foremost. Your class is just the toolkit of moves at your disposal. Some people were super hero's with their class, while most were just common soldiers.
How good you are involves your approach to your class, not the class itself. That's because at 60 Blizzard treated you like an individual player, and not a mere clone of everyone else.
It was a finesse game, a thinking game, a game about your determination to grind and gear up. That's the only measure of your viability. Any class at 60 is yours to shape, refine, and improve upon as best you can.