I wanted to like this game. It's a real underdog but I couldn't finish it.
It's got huge, aimless open levels, ragdolls, fun guns, but it irritates more than anything else. Same reason I never finished Black. Too hard, not enough checkpoints. Only here it's got regen health so you playing bad means you die instantly rather than you have to play cautiously until the next health kit.
So I blame regen health. Plus it isn't as innately fun as Black and as unique and striking as the environments are they feel like a big featureless copy-paste blur of Halo-esque or OoE-esque proportions.