I've had Monster Hunter Freedom (PSP) for awhile, and enjoyed playing that last year. So far I'm finding Monster Hunter Tri to be largely a big-screen console version of the PSP game... which is not necessarily a bad thing. The big addition is underwater monster-hunting, and it works fairly well-- very similar to the underwater sequences of Syphon Filter: Logan's Shadow or Super Mario Galaxy.
The graphics are a little bit better than the PSP, and while I busted out my seldom-used Intec
One problem with Monster Hunter Tri is that it uses pretty much the exact same text box/font style as the PSP version, and it's way too small on the big screen. On the PSP it was ok, but on the TV, it should have been formatted better. Plus Capcom seems to want to have a bit more story content in the Wii game so characters talk A LOT. There is SO MUCH FREAKING READING in Monster Hunter Tri. Of course, no voice work other than a few grunts.
And true to the Monster Hunter franchise, the difficulty ramps up pretty quick. There's a lot to manage in this RPG, and it is a slow-paced game.
I'm not sure there's a lot to significantly differentiate it from the PSP games, but if you like the Monster Hunter games, this is a pretty much a sure bet. I'm surprised how closely it sticks to the formula.