It's not "OMG it's shiny and new!" It's "OMG it's not an MMO and I want to play something different." The Diablo games are not MMO's, they are Action RPGs and in this case a dungeon crawler.
A lot of people like the game simply because it's relatively simple and you can play at your leisure. If you are a person who plays casually, then this is a great game. Unlike MMO's you can log on once a week and play by yourself, or play with friends. There's a maximum of 4 people per "room" and the more people you have the harder the monsters become. As you progress there are harder and harder modes. You start off in Normal Difficulty, then Nightmare, then Hell, then Inferno. So in essence you can play through the game a total 4 times getting progressively more difficult each time.
If you want to take it to the next level then you can play in hardcore mode. This hardcore mode is not the carebear hardcore mode, when you're character dies, it's gone, you can't open a ticket and have a GM reload it. Also, no amount of crying on the forums will help you either, so most of the Rift forum community should stay away from D3 hardcore mode.