I read these threads with some amusement. There's some people who are offended by things that wouldn't offend me, and others who may even be offended by what I do (I've been known to, horror, put sock-clad feet on a bulkhead; though it seems it may be OK to some people, because I usually keep them below seat level).
Anyway, here's a behavior I found pretty irritating, and I think most people agree: If you're sharing an armrest with someone, maybe you ought to keep your crap on your own side.
Sitting next to Mr. 1C today, who, for whatever reason, needed to have at his disposal at all times, his iPhone, his blackberry, his iPad, and his laptop (ok, not the laptop). Every time I lifted my arm from the armrest, another portable device, power adapter, or assorted gizmo would end up where my elbow wants to be.
I mean -- as much as people can b**tch about the horrors of a 22" wide seat, the armrests are a good 6 inches wide -- can't you please at least pretend like you're trying to keep your stuff on your own side?