Note that there are 8 souls for each class, but that you can only have 3 active in any build. While technically this means that you've got 336 possible combinations of souls per class, the reality is that there are some obvious synergies, and some combos that just don't work.
You also get 5 (or is it 6 now?) different specs that you can switch between pretty easily, consistently cheap respeccing, and decent gear re-use between roles (at least until 50).