091011 - THE ORIGIN OF THE KEANU-CAN'T-ACT BANDWAGON
1) Sometime in the early 90s, Keanu gained a very bad reputation among the media because he was extremely shy and often refused to be interviewed; in the rare occasions that he was, he usually either gave non-answers or spent the whole thing looking as though he'd rather be elsewhere. This frustrated a lot of journalists trying to get a scoop on the newest rising star, and some took it out on him by slamming his acting. Prior to this, Keanu's 1980s films received almost universal praise from film critics. He was hailed as the new talent to look out for, called the 'new de Niro', and similar things. Then the interviewers came knocking, Keanu shunned them, and suddenly the praise dipped drastically.
2) Coincidentally, around that period Keanu gave one of his career's worst performances in Dracula. It had been the last of a non-stop stretch of films (Bill & Ted 2, My Own Private Idaho, Point Break) filmed in 1991 with next to no breaks in-between, and he basically had been exhausted and far from his top acting form; Keanu admits this in interviews. But this just provided more fuel for the media slams.
3) Laypeople then read these negative comments about Keanu written by film critics, and concluded that if such intelligent, discerning writers thought that Keanu was a bad actor, they were probably right.
4) And so the bandwagon grew. People who disagreed were laughed at and accused of having lousy taste because 'everyone' thought that Keanu was a bad actor, and if you didn't, there had to be something wrong with you; if you were female, it meant that you were hopelessly infatuated with him, and if you were male, it meant that you were secretly gay for him. Most people did not like being thought of as such, and so joined in the bashing. It was the cool thing to do.
5) And several years later, this basically blossomed into Keanu being so intrinsically associated with bad acting, even by people who, when asked, haven't seen more than one or two of his films - sometimes even enjoying them. It's become so ingrained in pop-culture that most people don't even question it any more. When someone wants to make a joke involving a bad actor, they usually whip out Keanu's name.
Sure, he's far from the best actor out there, but I'd consider him at least average, and there are many other actors much worse than him who don't get half the flak he does.