YouTube is an amazing site. I spend a lot of time there browsing mostly music and training videos. I am impressed with the number of features they have introduced over the years.
They just released their latest feature that allows you to edit video online using the Youtube Video Editor.
I would have put Youtube in the Platform as a Service (PaaS) category a long time ago with their API and embed tools.
However with the ability to edit videos online they truly have graduated to a mature cloud video platform. Why would you build your own when you can use YouTube.