Second Life isn't a MMO as it is a Game: Really, it's just what ever you make in it.
As for if the MMO will be popular? I wouldn't say that Realism is always a good idea. It's just bad game design in the end. Think about your end user's experience, what makes a game fun, and also the long term.
There are a few reasons I can give that can predict how long a game like this will last. (not very long)
1. Always assume that a third of your game's population on MMOs will be jerks. This means that in a realistic game, murder, theft, and all sorts of vices will be easily do able by players without any real recourse. Should a player be playing the game longer than everyone else, you run into the situation where the game is dominated by trolls, and players whom are Uber Powerful. See WoW, and ALL online sandbox games. After your first few hours, the game is already frustratingly annoying without some guy running down the street with a shotgun popping you in the face while you are at work.
2. What makes the game fun? If there is realism in the game, then what separates it from real life? What is actually keeping you glued to it? Without anything that defies normal logic, the game won't really be entertaining after the first ten hours.
3. Realism is just a hype. "DUDE! This game is like super realistic!"
4. Pay monthly fees for a game that emulates your life? Or get paid hourly to play life?
Oh and lets not forget the costs just to make something like that.
But hey, if you really want to shoot for something like that, look into OpenSim. It can get you started.