Originally Posted by ajk
Actually YouTube won't convert 25fps material to 30fps - it will stay at 25fps. 30fps is just the maximum and anything above that will be converted. So the smoothness of those movie trailers and things like that can be displayed relatively intact. 50fps games can't be, of course.
Depending on exactly how it works, there might be interpolation of the frames, so it would be more like 30fps + *proper* motion blur which would give a smoother result.
If so, a game orginally rendered and captured at 50fps would end up in motion on youtube looking smoother than a game captured at 30, perhaps not even noticably different to the original.