Personally, when it comes to maths, my pet hate is Wikipedia.I hate online videos for almost every single subject. They either go far too fast to keep up, or more often too slowly, repeating the same point 3 or 4 times over in different ways, and I'm desperately wishing for the presenter to just get on with it.
Give me a blog explaining it any day.
Their mathematical articles are very user-unfriendly: too many equations, and not enough explanation of what those equations mean. And I say this as a maths graduate who usually understands those equations (unless the subject matter is very advanced ).