1) Nintendo always has the best first party games, every generation, since 1980s. They always deliver the best games albeit at a pace not many are satisfied with.
2) Couch co-op is still the best way to play games and Nintendo is the only first-party company still making couch co-op games consistently.
3) Off-TV gaming/Remote Play is one of the few actual next-gen features. Granted, the PS4 has this as well, but requires a ~$150-200 accessory to accomplish it. Wii-U does it right out of the box - though, arguably not as well as the PS4-Vita since you can't remote play over the internet (which is amazing for sneaking in some gaming at work).
4) There are now enough games to warrant the purchase of a Wii-U (Smash, Hyrule, Bayonetta, Super Mario, Mario Kart, Donkey Kong, Monster Hunter, etc.). There arguably are not enough first-party exclusive games to warrant the purchase of a PS4 or Xbone right now.
5) There are a ton of amazing first party titles coming down the line (Captain Toad, Kirby, Yoshi, Zelda, Dragon Quest, Xenoblade, Mario Party, Mario Maker, etc.).
6) It's the only system with backwards compatibility at the moment. And a ton of those old games are still amazing today because #1 and #2.
7) Even with the Black Friday deals, it's still the cheapest console (but only barely).
8) It's the only system with free online play.
That's pretty much it.