With it comes more monsters and subspecies (granted those were in Portable 3rd which we never got officially), the return of G Rank (level of difficulty higher than High Rank, originally shown in Freedom Unite), and new things to enjoy.
If you've not heard of Monster Hunter before, it's made by Capcom, the same ones who make your Street Fighter games. Monster Hunter is more popular in Japan but there is enough of a demand for it to be here in the US and Europe, the problem is Capcom derps about and doesn't pay attention to us. The only reason we got Tri on the Wii is because Nintendo did something good and pushed to release it themselves, and it ended up being one of the Wii's best sellers (even bundled with CC Pro package can still be bought!).
Just look up Monster Hunter Tri on youtube if you need to get an idea of the gameplay you will see in Tri U. Basically you have to actively dodge everything a monster does (solo play or up to a party of 4 online for a hunt), attack its weak spots (there are breaks and things you can cut off like tails!), and so much more than I have time to actively go into right now (gotta get out to college..ugh math). Also the MH musics are just epic, especially for Monsters like Lao Shan Lung and White Fatalis (older games like Unite before Tri) or in Tri itself, Deviljho, Alatreon, etc....
If you want a game where your own skill determines how good you do, look up the Monster Hunter series and fire up your Wii U's (or Wii's again if you still have Tri itself, servers close April).