So does the cross compatability work with EVERYTHING?

^ The guy in this video talks about how you can play levels from Xbox creators even if you have it on the PS3. So does this work with Wii U too? He keeps saying PSN and Xbox Live, but then again he says it works on any of the connected consoles. Does connected consoles mean Wii U as well?

yes, absolutely

Absolutely. I feel that the only reason that Nintendo Network is never mentioned as it is a fairly underused system that is only hitting it's stride with the Wii U. Xbox Live and PSN is very much set in gaming speak whereas NN is just getting in there.

Would you be able to play multiplayer with other people if they're using a different game system than yours? Like if I'm playing on the Wii and a friend is playing on the WiiU, would we be able to do online multiplayer together?

^ Nope, sadly.

Aw man, that sucks. Oh well. It's still worth it.