WP does not support flash that is why most videos will not play (im talking about streaming). However if you can find videos that support html 5 then you will be able to stream. In the current browser it's kind of hard to get full screen playback, but if I find videos that I can stream, I copy the link and use an app called Save & Play to download the videos to my phone and watch them from there..
It's a crude workaround, but it keeps me happy. There are one or two Flash video apps in the store, but I don't recommend them.