Cost is the starting point. Make an educated guess as to what Nintendo can reasonably charge for a new device/hardware platform.
I would not be expecting a lot of exotic or specialized parts in the interest of keeping R&D as well as BoM costs within that MSRP comfort zone.
So Nintendo is likely to use either mature or inexpensive tech, but in a creative way that results in a device that is different from anything on the market, but not from a performance perspective.
If the NX is some type of portable device that can be plugged into home entertainment set ups (HDTV), the specs and performance will take a hit at the expense of size/portability.
At this point, it almost sounds like a gaming tablet with traditional controls and an AV out port. Then the question becomes what type of package can Nintendo produce at portable console prices and still have acceptable performance. $300-400, closer to if not $300 assuming Nintendo wants to keep their core children's market on board.
I'm really curious at this point because solid NX information is really not there at this stage in project development. I don't know what it is, what market it's competing in, etc. Apparently, it's not in the traditional console market since Nintendo knows it can't compete with the PS4 in that arena, even if they chose to do a traditional $400 console utilizing whatever type of specs $300-400 will buy when sourcing components in 2015-2016 as opposed to the PS4's 2012-2013 time frame.
I'm sure clearer information will be made available as the product is closer to launch, but clarity would probably be a better policy than mystery if they are trying to carve out a future market for the NX rather than keep potential consumers confused.