It's a story driven RPG, the middle part in a trilogy. Games such as Fallout, Fable or Oblivion are more stand-alone games.
When you look at other WRPGs on the 360 the most that one sold is 3.4 million for Fallout 3. (Fable 2 sold 3.9 but I saw it bundled quite often) So WRPGs don't seem to be selling much more.