Great site, largely horrendous community
YouTube is a marvelous site that encourages creativity. However, please beware the comment sections and many videos. YouTube Comments are basically almost the scum of the internet. It is full of misinformation, trolls, racists, bigots, spammers, conspiracy theorists, etc. If you are to let your child or teen on, please turn Safety Mode on. I swear that reading those comments has made me lose faith in humanity. However, it is a good idea to enable comments for how-to videos, since they usually can contain very useful information from other well-meaning members. If you are teaching them tolerance, turn off comments for cultural videos, especially Africa and China videos: They are full of hate wars and such.
Aside from the community, the site is actually pretty good. It hosts a lot of great info and even has a copyright system that on many videos allows you to host copyrighted material while they place ads on your screen. However, be wary of uploading most copyrighted content, as the company can give you a strike for it. 3 strikes and you're done. The video collection can also be quite explicit, hosting videos of graphic violence, and even borderline pornography. Also be wary of Animated Music Videos, parodies and fanfiction of kids shows: Many of them, especially My Little Pony ones, contain a lot of taboo subjects, sexual content, gore, etc. Another thing to be wary about are "screamers". These videos are frequently disguised as relaxing, brain teasers or subliminal message revealer videos. The best bet is not to let young children watch such videos before you watch it first. For teenagers, it can give them quite a thrill and a laugh as long as they're not squeamish.
For ones who don't like reading big blocks of text, I'll provide a summary. It's great for everyone as long as you choose the right videos and avoid most of the comments.
What other families should know
Too much violence
Too much sex
Too much swearing
Too much consumerism
Too much drinking/drugs/smoking
Safety and privacy concerns