The issues that caused it to largely fail.
I think the Wii-U is a fine system, too, but that doesn't stop it from barely getting turned on except to play Wind Waker, Super Mario Galaxy, maybe Tropical Freeze, and coming March, Breath of the Wild. Now I know this'll differ from person to person, and that the Wii-U was mainly a companion console, but even with that the console was barren.
The swiftness with which the Switch was announced seemed like a response to the floundering of the Wii-U. So I'm sure most people -- myself included -- expected Nintendo to come out with a console and/or line-up that would restore faith and indicate a real change. But this just seems like it's going the same damn route, which didn't exactly work out for them previously.
From the first-party side nothing intersting was announced that I can't play on the Wii-U, and I'm sure I don't have to expect any sort of third-party support. So what we're getting is another strictly Nintendo machine for playing the same old stagnant IPs.
Again, I would've gotten excited if it allowed me to play 3DS games on it, sort of showing how dedicated they are to the whole home console/handheld merger, and thereby revealing something that was genuinely different than before, but no, it's just Wii-U2.