Extremely limited multiplayer and slightly less accessible gameplay. That's all there is to it.
Besides, not every game can be a 20 million unit mega-blockbuster. I'm sure Nintendo will keep delivering highly polished 3D Marios as long as they sell like cute little baby blockbusters.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.