The controller. The controller's design is horrible, and if they made other kinds of controllers that are completely compatible with the system then that would be a great thing to happen.

The external hard drive compatibility. As of now, my external hard drive works fine on the OUYA. But if I'm trying to play ROMs on my emulators from the external hard drive (NES.EMU, MD.EMU, Snes9x, etc), they have a terrible audio to video lag and it's because of it being loaded from an external hard drive. If they could do a firmware update to fix this, then that would be an immensely good thing. Also if they could allow more kinds of external HDDs to the fold to be used on the OUYA, then that would be great too.

More kinds of controllers. If OUYA dev could allow more types of bluetooth controllers to work on the system, then that could very open up more ways to play games on the system. For instance I bought a Nyko Play Pad Pro for Android in the hopes it would work pretty well on the system. It worked up to a point; it could access menus and apps just fine (XBMC and such) as well as play emulators fine, but it can't play OUYA games and if the bluetooth light goes out on the controller, it will automatically back you out of whatever software you're currently running and push you back to the previous menu. It gets annoying seeing controllers you hope would work and love suddenly just lose their luster afterward for not working completely. Also if new controllers can be added, it'd be good if they can be mapped to have the same button mappings as the original OUYA controller to avoid any sort of incompatibility issues through the options.

More responsive UI. As of now, the menus could be a lot better. There's a hint of audio lag whenever I move around in the menus, but maybe that's just me. If the menus could also be customized (like someone mentioned having backgrounds and such), then that would be really really cool. That'd also add the incentive to make the console more yours, since it can be "hacked" as the OUYA team said.

A better OUYA team. Let's face it. The OUYA team blows. They never give honest answers to questions and were keeping us all in the dark on when our units were being shipped to us. I also heard they weren't shipping to Australia or something too, and that made me mad at OUYA for those people. They deserve their units just as much as the rest of us did. You suck OUYA team. Get new management.