[Izkata]

Eh, I'm not entirely sure I would have. The effect is near identical to when you have one person behind another, pretending to be the front person's arms and hands - the motions just don't match up right.

Maybe people who use faces more than full body language are more affected by this particular one?

[d2v]

I kind of agree, but it's hard to separate knowing that it's fake from my reaction to it. If this were presented without that context, I might feel like it's off, but not necessarily question it, especially if it came from a source I believed to be legitimate.

[m-watson]

I also think, on top of all your points (which I agree with), keeping it shorter would make it more convincing. I think with the current usage of social media, short clips created through some synthetic means will be harder and harder to identify.