In a Neogaf thread (link), user UncleSporky, has kindly summarised a few details that were given out by Minecraft: New Nintendo 3DS developers on their dev stream.

-max world size is 2016×2016, which is not bad…for comparison small worlds on Switch are 900x900ish and largest are 3000x3000ish
– based on Pocket Edition 0.15.4, which launched July 2016, has every feature the game did as of that release
– due to above, no The End yet, but does have the Nether
– redstone contraptions work as expected, they had memory-eating bugs with them at one point but got them all squashed
– they plan on lots of updates over the course of the next 6-8 months, for example targeting 0.16 and adding ocean monuments and associated blocks
– no cross play or world sharing, this is a totally new and different build of the game unique to 3DS
– no multiplayer at launch, they want to put it in eventually (only among 3DSs)
– 3D depth will be added in a patch very, very soon, 99% ready
– mouselook with c-stick including invert Y-axis if you want, other options like view bobbing off, auto-jump
– best unique feature: map is always active on the bottom screen! no need for a map item or dual wielding it or anything
– comes with standard textures, plastic (looks great on 3DS), and city, plus many character skins

It would seem that the New Nintendo 3DS version of Minecraft is going to be a little bit lacking, if not for the upcoming glasses-free 3DS feature. The lack of the upcoming cross-play will be a huge disappointment. However, the developers have promised future feature updates for the game so hopefully, there will be more feature parity when compared with the other versions in the future.