Hadn't heard of this game, but reading that they'll let you pick and choose (mix-n-match) means that play balance will always allude them. There will likely end up being one specific set of abilities that will be uber and the rest will be greatly underpowered. There's a reason most MMO's don't do this method.
Although, the idea of having multiple roles is interesting, but it feels like that's only a way to not have to have so many alts. Anyone following this game?