Yep, the technical shit is one of the main factors for me. It just doesn't flow well, getting caught up on pavements, can't jump off a small ledge, the cover system bugging out, there are too many niggles to ruin what should be a fluid experience.
The fact that every bad guys is the same and can take a gazillion bullets to the face just adds to the feeling of being bored. If somethings going to take that many headships to kill, then it needs to be a 30ft tall fire breathing dragon shaped robot, not a dude in a bulletproof vest. That's just retarded.