MP3 would have sold better if it had online play.

[Grey Acumen]

I'll throw in my lot that online multiplayer, or for that matter, any multiplayer at all, would have been nice.
I played MP2 for the gamecube against some friends and it actually got to be kinda fun, but I would have liked if I could have used the wiimote for that as well.
I don't really see how anyone can argue that Multiplayer would have been another selling point for the game, as most of the FPS crowd is currently on the 360, and has come to expect online play from FPS games.

Still, the biggest problem that I saw for Metroid Prime 3 is how it was advertised. that "Wii would like to play" bit is nice for those casual gamer crowds, but Metroid Prime 3 should have targeted the hardcore gamer crowd and basically spelled out "This game is the most immersive FPS ever! Buy it, even if you have to buy a Wii in order to play it"