A little bit underwhelming hardware-wise. Extra joycon L/R don't seem like they'll factor in much in most singleplayer docked or handheld settings. (I guess those aren't new buttons..
.) The mouse thing (if that's what they're showing) is somewhat interesting and helps to keep touch controls relevant when docked. Top USB port, sure. Backwards compatibility is great news. Screen is...?
As always with Nintendo I think it comes down to the games. Mario Kart 9 hasn't blown me away just yet, but we've barely seen anything at all so...
A mouse mode could be extremely useful for things like Civilization or Fire Emblem. Of course, the touch screen is also great for that and Three Houses and Triangle Strategy didn't support it.
It looked like they were mimicking cars to me, not mice. Not saying that it wouldn't be a cool concept if it has an optical sensor. But they didn't show any visual feedback to imply they were controlling any kind of cursor. If that was the implication, it's obvious it was a rushed marketing piece.
Please excuse the screenshot of a video but this looks a lot like an optical sensor. I’ve seen a lot of internet “experts” covering this and the general consensus for now is optical sensor too.