If you want to play something similar to Diablo to know what the playstyle is going to be like, I would suggest downloading the free demo of Torchlight, which you can find on Steam. It's not as good as the Diablo series, but it's still a pretty good idea of what to expect. You just have to pay the box fee for the game. There is no monthly subscription afterward. And although it is online and you can play with other people, it isn't an MMO in the way that WoW is.