My brother was playing Tri yesterday (because he doesn't have a 3DS nor PSP) and I realized how much I miss 3rd Gen monsters.
I like Khezu, but Gigginox is infinitely better and way more exciting to fight. Khezu feels like a punchbag you have to be careful every once in a while in order to not get caught in its lightning attack. Gigginox, in the other hand, is way more mobile and has interesting and fun gimmicks.
As much as I like Kut-Ku and the memories of being triple carted by it in MH1, Qurupeco is way, way, way better. Besides being a completely different monster with unique animations and attacks (unlike Kut-ku being a watered-down Rathian), Qurupeco brings more monsters into the mix and it feels great to kill the monster it called!
Some monsters could have benefited from the new mobility enhancements and the new terrain design (Nargacuga, Barioth), and others could have been in the game without been modified at all (Nibelsnarf, Volvidon, Arzuros, Akantor). Hell, Royal Ludroth was in P3rd, and there wasn't underwater fighting in there!
I know it may be a controversial opinion, but I really, really hope they bring back underwater hunts in MH5. Maybe they would need to refine the controls a little bit more (I never had a problem underwater, but it's a common complaint), but imagine fighting vertically, horizontally AND underwater! Imagine the potential of a new, "all-terrain" flagship Elder Dragon!