The storyline is pretty short, especially compared to DII. Blew through the game with my monk in less than 10 hours on Normal Difficulty. Fortunately, Diablo has never been about the story, but more about the phat lootz. If you accept this, you'll be fine!
The only thing I really don't care for is that your friends can just randomly show up in a game, if you have them on your list. Normally this isn't that bad, since most of my friends aren't complete morons when playing cooperatively, but their friends can now join the game I'm in, significantly complicating the situation.