So far, my pros and cons of what has been speculated:
Pros:
Power - sounds like an incredible leap over the 3DS, and powerful enough to actually be worth using on your home TV
Hybrid - being able to use your 50' screen to play games while at home is one hell of a bonus feature for a handheld.
Screen Size - I always wanted this to be closer to a tablet than a phone. Pocketability doesn't matter - you can't replace people's phones as their pocketable devices so don't bother trying. A 6', 720p screen is all I want and need from a handheld device. MK8 ran at 720p and it looked amazing. Put that on a 6' screen at minds will be blown.
End of Dual Screen - Cost too much battery and processing resources for what it was adding to games.
Unified Nintendo Library - Great games all the time, end of droughts, Hallelujah.
Cons:
Detachable Controllers - I have no idea how they'll work, but I'm concerned that they'll feel bad in the hands, that they'll be flimsy, that they'll make the hardware itself look cheap due to whatever method is used to attach them. I'm conerned we won't have as many buttons as we should due to the requirement that both sides be usable on their own. I'm concerned that this is the latest example of an attempt by Nintendo to push local multiplayer that ends up sabotaging their ideas. Local multiplayer should NOT be the focus of a portable device. People are all about the individual second screen experience these days. If you want local multiplayer out of the NX, hook it up to the TV and use a second controller sold seperately. (Or a BC Wii / Wii U controller etc)
Proposed Audience - The above makes me think they might be skewing very, very young with this thing. Younger than usual, even. Simpler controls, local multiplayer on a tablet sized device... sounds more for 5-10 year olds and less what I'd want from a device. This is just a fear more than anything I substantially believe... just throwing it out there.
Industrial Design This thing needs to look sexy if it's to sell well. it needs to look 'normal' enough that people can safely use it in public without looking like they're playing with a toy, and preferably like their using just any modern tablet device, if a bit specialised. The detachable controllers make me think that this will be impossible, and this thing could be a bit of a fat monstrosity compared to the sleek electronics we're used to from other companies.
All of this is useless speculation but it's where my head is at right now. By and large, other than the controllers I think it sounds like everything I've ever wanted, and I've learned not to really doubt Nintendo on when it comes to input devices.