I said "one thing"
There are obviously plenty of other factors. A good example is Mario Galaxy vs NSMB wii. One is an audiovisual cornucopia with a two piece controller, 3d spatial puzzles, peppered cutscenes and dialouge throughout, and a rather tame co-op element. The other is straightforward intuitive left to right obstacle courses with an optional "be the biggest prick" mode for good laughs with no story elements to speak of other than the premise.