yeah, unless the online is flawless, i really don't want online co-op... that goes for all platformers too, not just Mario.
General thoughts on this game: I'm excited. I know coins/"gold" are the hook, but after reading lots of E3 impressions saying how hard it is, how the good level design is, SMB3 refernces, and a slightly new take on backgrounds (etc.); that's really all I need to hear to be excited for a Mario game, or at least a NSMB game. The coin thing is definitely interesting, and everyone was surprised at how addicting it is, so that'll be a bonus for me (along with any other goodies, i.e. StreetPass, Coin Rush, etc.).