Seriouspost: A talent system is just a more elaborate straitjacket. If your game's hardest content is co-op, people will either go for the optimal choice or get nothing but grief from everyone else if they don't.

A somewhat better, though still flawed way to add customization would be to add more flexibility to cross-class skills. There'd still be "optimal" choices, but if much of the overlap was redundant then it'd give players a way to tailor the way their character feels to them.