That's extremely inaccurate. It is based on Monster Hunter Tri (which was for the Wii), but it has maybe thirty new monsters, dozens of new features, hundreds of new weapons, a few new areas, cross-system save transfer and multiplayer (Wii U and 3DS), and a handful of new characters.
In terms of graphics, it's rather mixed. While it's got native 1080p and many of the new monsters/areas are very well done, almost all of the old stuff is just hi-res textures. You can still see the blocky outlines on the old stuff. It's not a huge deal (the massive amount of content is definitely worth it), but it would have been cool if they had redesigned it.