Here's my personal list:
1. Third-party support: If I'm going to consider dropping $300+ on a console, it absolutely needs to have more than just first-party games on it. Sports games must be represented, as well as other popular franchises that I enjoy playing.
2. More data storage space: 32GB is a joke, and adding an external HDD only served to add to the cost of the WiiU... which was a major reason I rejected it. I'd like to think that more space will be included with the next platform release, but this is Nintendo we're talking about here.
3. "The Killer App": WiiU was the first Nintendo console that I never bought, largely because there wasn't that "killer app" that I just HAD to have... like there was with every other box. NES had Super Mario Bros. SNES had Super Mario World & Final Fantasy II. N64 had Mario 64 and the best version of NBA Hangtime. Gamecube had Rogue Leader. Wii had Wii Sports. WiiU never had that game/games that I had to have. Maybe NX will.
4. Achievement/Trophy System: No, Miiverse doesn't count for me... and yes, I know that many GAFfers despise Achievements. Still, I've come to expect them in my gaming experience. Sony, MS, Apple, and Steam all utilize a system of Achievements. They add replay value for me and, depending on how creative the Achievements are, they can get me to experiment and play games in a different way.