To answer your specific question:
I'm waiting for three things I think.
Anything interesting either by virtue of its novelty or its execution.
Anything interesting based on its relevance to the kind of juggling I'm doing.
Anything which will confirm their skill level to me.
I don't believe number 3 should have anything to do with how enjoyable or valuable I find the video but when I watch a new juggler for the first time I'm definitely judging their skills.
I guess the difference between watching an unknown juggler and a known one is that I'm not as concerned with their skill level.
Of course, sometimes a video is just enjoyable I'm not really waiting for anything. For example Lewis Kennedy's videos are always a delight no matter what he's doing.
tl;dr: Anything new, old but beautifully done, relevant to me, impressive or just fun fun fun. (I guess that's five things.)