For me it was unregulated spam.
For example, the mfg omg servos would give away free servos provided the user posts a video link at RCG. And so there were tons of new members who made 1 post then never showed up again. Search for the user omgservo and he has made tons of posts - every one promoting his product and not one making a real contribution to any discussion. RCG allows it, RCU deletes spam and closes spam accounts.