There's a lot of great stuff on Youtube for kids (genuinely, whole-heartedly, terrific stuff; I was amazed), but there's also a lot of very, very weird and sexualized and violent content. Targeted at young children. And unfortunately, the 'recommended' sidebar is a little bit too eager in sending kids down those paths.
I see some comments comparing it to 'video games ruining society' and 'violent cartoons'. No, it is not at all the same thing. I have no problem handing the ipad to my son and allowing him to select a show on Netflix. Youtube crossed my 'wtf creepy' threshold a few too many times.