i gotta agree with buddy here.
the main problem is combat fatigue. it's the same reason i can't play halo or killzone or them other shooters. it's too repetitive, because the pacing is always focused on combat. some people like that, but one of the things that made half life great was that its pacing switched between puzzles and combat. it's the balance between the two that makes for a game that i can sit for hours and play, whereas neither of the two alone are fun when prolonged.
pacing is one of the most important aspects of design, and it seems very few people in the industry actually focus on it.