I am a little bit perplexed to be honest. I know NX hasn't been officially revealed yet and we are simply going off rumors (although it seems like this is the real deal since multiple sources are reporting it). However, it is bizarre that Nintendo seems to be making an evolution of the Wii U, even though they have stated multiple times that the NX will not be related to the Wii U and will be a brand new concept. Having a detachable controllers is a new concept but it is kind of funny that it is basically this:
Why specifically would it be an evolution of the Wii U?
Going by the rumors- this brings together design philosophies from many of Nintendo's past consoles.
The ability to game on the go is obviously nothing new here, but they're returning to a one screen experience like with the game boy.
The two detachable controllers seem to serve two functions - one person holding them both for more elaborate single player experiences, like with the Wii. Or two people holding them separately, SNES style, for simpler, mulitplayer game experiences.
I'd imagine the detachable controllers are an option for when you're on the go, but required when playing at home, when the hardware is docked.
I don't know if there really is going to be a whole other gimmick layered on top of this, beyond allowing a traditional but flexible and fluid way to play Nintendo games. The main question I have would be how much software Nintendo will be pushing purely as a touchscreen/tablet experience. Touch on the tablet and IR use on the big screen would seem to overlap pretty well functionally.
The main hooks of the Wii U were two screen gaming, asynchronous multiplayer, and limited off-screen play. The only thing the NX is really borrowing from the Wii U is the bigger screen, tablet-like functionality.
From a marketing standpoint, the messaging is simple: Play your games where ever, on the small screen or the big one. It doesn't need to be more complicated than that.