I don't know what it is like for the other classes but in reference to what Kurtis Seid said regarding different types of classes - For Wizzards anyway there is various combinations you can choose - maybe not as extensive as d2 but there are some variances. Like arcane based or frost based, and you can mix up your spells to form your own mix not just from one tree like in d2.

For the moment I am 56 just started act III of Hell and will continue to play till 60 and finished inferno. Then will play on and off till PVP comes out and/or slowly level another class.