PDA. Especially when couples hold hands together in public in narrow alleyways (or the Great Wall of China, for example), while walking very, very slowly.
And I mean the type who you can tell a really basic point to over and over and they STILL don't grasp it! (Like my sister being unable to comprehend that the reason the files in her 'my music' were the same as he MP3 player was because I copied and pasted them)
Stupid (old) people who insist on driving under the speed limit in the fast lane. Stupid people who drive side by side on two lane roads so you can't pass. Stupid people who don't use their blinkers.
Bastards who state an opinion as absolute, undeniable fact because someone sold them into it (a relative/spouse, a church, the first biased reporter they see talking about the subject, an online chat group, etc.).