The "being watched" feeling is the big one for me. I don't mind wearing headphones because I love listening to music and I don't listen to it that loudly, but the sensation of feeling someone's eyes on you (to catch you on the split second you've refreshed reddit or facebook or something) is incredibly profound on the time it takes to zone into zen mode.
Anecdotally funnily enough, the opposite seems to happen for me when I am pair programming: each person keeps the other in focus, but obviously this only works when both people have the same objective. Not really related to "open office" issue, but thought it was relevant.
Or maybe I'm just a filthy procrastinator...