Well, as others have said, money. Without third-party support and very little diversity with first-party titles, Nintendo home consoles will always be "second consoles" for me. That means a sub-$200 price before I would even think about it.
Second, proper Zelda and Metroid titles. I don't care much for platformers, which leaves out a lot of Nintendo IP, but an old-school, gamepad controlled Zelda would work wonders.
Third, more new IP aimed at an older demographic. I grew up with reading books as my primary hobby, so I love complex plots and interesting characters. Throw away storylines and flat characters do nothing for me. They can develop Metroid and Zelda, why not other games in the same mold?
Fourth, more RPGs and SRPGs. Fire Emblem is why I want a 3DS. The Project Rainfall games were the only reasons I wanted a Wii. Turn the Wii U into the definitive RPG box and I'll buy one eventually. X kind of helps but I don't really like mecha very much and I thought Xenogears was overrated.
Fifth, seriously, Nintendo needs to get up-to-date with their online infrastructure and account system. It's beyond lame that they're years behind on those fronts.
