When gamers skip through the tutorials/in-game tips of games they play (especially Let's Players) and then hit the brick wall that the tutorial was telling you how to get around. It's annoying because, for the non-Let's Players, they end up asking on the forums how to do something that, if they had bothered to actually READ the tutorial, was perfectly explained to them... and in the case of Let's Players, it creates an incredibly frustrating and not fun at all moment where they keep throwing themselves at the problem, complaining that the game didn't explain anything to them (which, if they had just shut up for a moment and either listened to the Helpful Information Character talk, they would have been through in two minutes) and wondering what to do.
Also gamers who complain about how games "handhold" people. See above for why games handhold people now. And the sad thing is? A lot of these "dumbass gamers" are actually "hardcore" (pffft) gamers who have been playing almost as long as I have, sometimes longer :-/