Why is "almost" so aggravating?
These stats from eMarketer show that web surfers find advertising parading as content misleading. Sponsored video ads that falsely appear to be content (read user-generated or genuine) top the deception list.
This may suggest that the more advertising fodder mimics "real" content; the more negative the audience reaction. In the field of robotics there's a similar phenomenon: the uncanny valley.
Uncanny valley explained
A human observer feels more empathy for robots as they become more life-like up to a certain point. Stuffed animals are cuter than industrial robots. As the robot's appearance approaches the most human-like, empathy quickly gives way to strong revulsion. People are creeped out by robots that look "almost human."
Some draw upon human evolution to explain the uncanny valley. With advertising, it may not be as complex. When someone expects interesting content and gets an advert instead, they likely feel harassed and deceived.
Photo credit: Tom Woodward via Flickr