My first argument is that free online and off-screen play makes for a great second-console experience. It also features the Mii-Verse, which a is a great tool for sharing things about games you've taken to.
I will provide concepts of what you can get and back them up with multiple examples.
My second is that it offers an assortment of lesser seen genres, and other games that barely fit within defined genres, at the first and foremost of their exclusive line-up; most of which are extremely highly rated.
- Pikmin is an Adventure/RTS hybrid which is simply very improbable. It is a very unique title but one which has actually influenced games on other titles, namely Overlord - though it has a very different tone and pushes a more RPG tone based on that same concept.
- Super Mario 3DS World is possibly truly the only great 3D Platformer we've had since Mario Galaxy. Personally I couldn't live without this genre, which is part of why I'll always have a Nintendo console.
- Splatoon seems to be one of the most unique shooters we've seen in a long time (granted, Sunset Overdrive also realized the genre was crying out for some kind of re-imagining of it to it).
There are also a number of classic games you will have the opportunity to catch up. In addition to the virtual console their seem to be a number of titles, both released
- Mario Maker will allow you to experience countless variations of the classic Mario ideas that you may have missed.
- Legend of Zelda: Wind Waker HD is a wonderfully upgraded version of a game many argue to be one of the best of all time. While hardly the perfect open-world game by todays standards it still very effectively stands its ground.
- Backwards compatibility will allow you to experience any games from the Wii-era you might have missed. Remember that the Wii-U is perfectly backwards compatible with all Wii games.
Additionally there are a number of games that fit more in-line with traditional games. These can include games which belong to Nintendo, or which they bring in from outside.
- Metroid Prime is a very dark atmospheric first-person shooter series based on the Metroid franchise, and with gameplay that derives in many ways from the Metroidvania genre.
- Fatal Frame is another dark atmospheric first person game, though its a survival horror. It doesn't use conventional weapons as the enemies/challenges are ghosts.
- As you may know Bayonetta is a spiritual successor to the original Devil May Cry game which was extremely well receive, and the sequel (as well a seeming enhanced remake of the original that comes with) is a Wii-U exclusive.
There are also games that strive to be a more colourful and less serious than those on rival consoles, which can make for a very refreshing experience when played alongside other options.
- Mario Kart is a wonderful and wacky multi-player, which can be a fantastic game for playing over the long term. Playing with friends is great, but it is also an incredible palette cleanser between different games.
- Smash Bros. is reliably comparable to this. It is a very different game fighting game that is so different to traditional fighters that some people argue it is more of a multi-player competitive beat-em-up/action game.
- Kirby's Rainbow Curse looks to be very nicely stylized game which utilizes the Wii-U's larger gamepad to vastly improve one of the most popular DS games significantly.
There are also a number of very impressive seems games being promised at the moment. In there are two which, from what we already know, may easily prove to be as impressive as what is on 'The Twins' for a significant part of their life-cycles. After all Mario Galaxy managed to hold its own against as a strong contender for the game of the last generation for many critics.
- Zelda U appears to be combine Wind-Waker's style with more detail and higher vert models. This is likely to act as a short-cut to being one of the most beautiful games of the generation given how well Wind-Waker itself still stands up today.
- Xenoblade Chronicles X seems to be working with mind-blowing scales and so far looks to be the kind of game that will inspire amazement in the player over the nature of the world they're playing in.
I also feel it is important to tell you that, by getting a Wii-U you'll instantly have access to a number of very worthwhile games. Here are some commonly skipped ones I haven't mentioned yet (the Wii-U's reputation for having very few games hides the fact that a lot of people haven't had the chance to get its full library).
- Donkey Kong Country: Tropical Freeze is by all accounts one of the most impressive 2D Platformers in the last few years, and is an especially challenging games.
- Nintendoland is a collection of games, some of which are extremely fun and all of which are effectively the size of eShop games.
- Wonderful 101 is a unique twist on the beat-em-up genre and is quickly developing a major fan-following.
Two more little points I'd like to include for you. One is a matter of a port of a third-party game that is honestly a masterpiece of the game that gets even better on the Wii-U, and one is a game I begged for it after experiencing it as a side-game.
- Deus Ex: Human Revolution is incredible on Wii-U. The uses of the Gamepad are amazing and manages to be greatly enhance a game which, lets be honest, was never built for it. It made it very obvious why the dev-team wanted to make this port.
- Captain Toad: Treasure Tracker was begged for via Mii-Verse by myself and many players of Super Mario 3D World and Nintendo have, by all accounts, got plans for solid expansion and explorations of the concept.
There are a number of other promising games and rumors to keep an ear out for or an eye on.
- Yoshi's Woolly World
- Fire Emblem x Shin Megami Tensei
- Hyrule Warriors
ALSO - Star Fox.