I do think the Wii U was an underrated and under utilised console.
In terms of games, all I really need to say is that most of the exclusives are being ported to the Switch which I think says enough about the quality of its library.
Hardware though, I enjoyed the separate screen. For the Zelda games it was just very convenient and especially with Windwaker, quite emmersive to be looking down at the map whilst sailing the seas. Games like Zombi U really took advantage of the second screen, having the inventory system be handled in real time add to the gameplay and tone of the game. For something like Splatoon, something as simple as just being able to quickly tap on a area of a map and fly there was just a quality of life addition only the Wii U could provide. Or something like New Super Mario Bros U which, whilst I think that particular series is very boring, sterile and not worth my time, the option of having somebody on the gamepad interact with the game in a different way showed a huge missed opportunity for some really cool asymmetric multiplayer games.
There were some bad examples, a huge one being Star Fox Zero. I think this points more towards the fault with Nintendo that when a console has a gimmick, they HAVE to make sure all their games use that gimmick in some way, even if it's to the detriment of the game.
The Gyro controls were also amazing. I moved onto PC around that time for genres such as first person shooters and to this day, console shooters just feel absolutely awful...Unless I'm using the gyro option on a Nintendo console.
The backwards compatibility too! My lawd, I never owned a Wii so this made it possible for me to play through all the titles I had missed out on. Alongside that, just having the Wii controllers work on the Wii U was a huge thumbs up, allowing older hardware to be used on newer systems so that you can just use those old Wiimotes in multiplayer rather than spending another £150 on three new Wii U controllers.
I think there were a lot of ideas there with many of them making their way to the Switch. I do wish the second screen was still utilised though, maybe via streaming to the TV...And the touch screen as that's a useless feature of the Switch that raises the price but never gets used.
I loved the Wii U. Whilst realistically it didn't get as much support as it should have (there were HUGE periods of nothing to play) I thoroughly enjoyed the time I did spend with the system. Usually I don't have much love for the hardware specifically, it's just the middle man that allows me to enjoy games, but the Wii U hardware and/or what it could do actually brought quite a few things to the games that Switch ports are just not able to recapture. I could play any version of Super Metroid and have basically the same experience with any version, but you'll never experience the Wii U version of Windwaker quite the same anywhere else (well, when it does get ported again).
) on it which you can perfectly play without having to turn the TV on as well, as well as Game & Wario, some of the features of which mostly don't mandate me to look at both the TV and the GamePad at the same time. (Unless it comes to 'Gamer' of course. 
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