I think what disqualifies Sherlock for Slytherin is that he isn't power hungry (just attention-hungry), doesn't perceive himself as a person whose worth is greater than that of others (despite his blatant arrogance), and doesn't maipulate others for personal gain, but rather to further his true interest: solving riddles. I should add that the qualities I just ascribed to Slytherin are maybe slanted towards the negative but I think it's exactly lacking the darker sides of being Slytherin that makes Sherlock more of a Ravenclaw.