IMO, the only really bad thing about the Wii U was the name and all the confusion it caused among the public. The launch of the console was majorly mis-managed and mis-communicated. Nintendo showed off the controller and didn't make it at all clear that it was a brand new console, and its name just added to that confusion. Most people came away thinking it was some kind of add-on or accessory for their Wii, which they didn't really need. It was all a major marketing blunder that the Wii U never really recovered from and was able to find its footing.
All that aside, the console itself was fine. The back-compatibility with Wii discs and VC games was nice. MiiVerse was an interesting experiment. There was, as usual, pretty good first-party support (Mario Kart, Smash Bros., Mario 3D World, NSMB, Paper Mario, Pikmin 3, Donkey Kong Country, Splatoon, Captain Toad, plus Yoshi and Kirby games, Mario Tennis, and others). And there were some really solid 3rd-party exclusives at the beginning (had a lot of fun with Lego City Undercover), but those dried-up pretty fast when the console failed to take off.
Overall, I felt there were enough quality first-party titles alone to make it a worthy purchase, and I can't figure out why more people didn't. Overall, though, I view the console as so much missed opportunity. Missed opportunity for Nintendo to build on the Wii's massive success and properly launch a successor. Missed opportunity for people to play some quality games. Missed opportunity for third-parties to really exploit the system and it's unique gamepad - something that never really happened aside from some early creative attempts like ZombiU. The possibilities for two-screen play (separate map screens, inventories, functions, perspectives, etc) never really came to fruition. I could imagine an NFL game where you select your play on the gamepad. There were so many possibilities for gameplay that never happened because the system didn't sell better. It's just a shame.