For me the repetition killed the game. Every mission is a carbon copy of the last mission, only in a slightly different part of the jungle, or what have you. It just couldn't hold my interest.
So far, Fallout 3 is a great example of an "open world" game that can hold my interest. Although the jury is still out on it, since I haven't had a chance to really dive in very deep.
I do enjoy the tiered health system. Actually I think it's a fantastic edition to the series. But the whole idea of stopping to pull out a bullet, is an idea that although sounds great on paper, really just becomes repetitive and tiresome.
This game just failed to grab me and pull me into its world and I swear I gave it my all.