Obvious: What it looks like, whether discs or cartridges, hybrid or seperate console/portable units.

Hopefully they will show the price, and that it is no more than $300.

And a 3D Mario game that has vast quantities of exploration if possible, at least something more akin to Galaxy rather than 3D Land/World.

Region free.

You should point out that it will only be 3 minutes. Cuz there will be lots of sad faces. 

They need to show Vanquish 2, dammit.

-What the NX is. And explain it WELL. We can't have another WiiU situation. This is ESENTIAL for them.
-Show the games. They exist, so why not a trailer. Maybe some 3rd party too.
-They don't need to show specs, but they do need to tell us after the show. Price would be great too.

It's only 3 minutes, so at most I imagine is there will be the official name, a look at the system itself and what it actually is, such as a portable/home console/hybrid of both, etc and then maybe end with a bit of new footage of Zelda running on it as a tease, ending with an official full reveal date where they fully show it off, as this is just a preview.

1) Unified games library (every game plays both on handheld & at home on your TV). (keyword *every)
2) Be priced reasonable compaired to the hardware inside it. If its a tablet, not be more than 249$.

I think it is highly unlikely they talk about price today.

New games from Ninty and 3rd parties.
Decently powered - At least PS4 level.
Good price - 299$ is the highest they can go, especially if it's a hybrid (which kind of sucks since hybrid technology would obviously cost more).

They are not gonna show the specs, I expect they explain the concept of the console and they show BotW and the new Mario. Also a name maybe.