My biggest concern with whatever system they have is the fact that they just can't make games fast enough. With Nintendo you're always waiting around for games, and you have to hope that the one game they bring out each quarter is a game that you actually care about. They've been like this since the days of the N64. I got a N64 at launch, and let me tell you.... the first 10 months of that system it was a total waiting game for games to come out. It was wait, wait and wait some more. Sure, Nintendo would deliver some killer games here and there, but the wait in-between the games was super painful.
So regardless of what Nintendo is going to bring to the table, my biggest hope is that they have enough games to really give some variety right away. I'm talking about a new F-Zero, new Mario, the Zelda cross port and maybe a Metroid. They really need to come out guns blazing, but I just have a hard time seeing them actually do that. They always take too damn long, and have too few games. They make great games, no question, they have a magical quality about most Nintendo games that is pretty much Disney-like, but playing the waiting game really sucks. (early adopters of the N64 know exactly what I'm talking about...)
Also, they need to follow it up with a second wave of games. So, not only do they need to have a legit launch window lineup, but then they need a legit 2nd wave of games to follow. That's going to be the hardest part.