It feels a bit undecided on for whom it is, maybe even a bit like you are not 100% about this yourself. Throwing an a use-case or two would help.
Don't just show me what it does and how great it is at thet, show me my benefit.
If you focused on the Video more this could be: Remix your yt-channel: increases the views of existing videos and gives new viewers just the highlights.
Right now: Need to throw together the perfect mix for a party? Mix spontaneus or well planned from all of youtube, without advertisement interuptions and easy to share.
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Don't make me log-in before trying it out. Let me dive in to an example playlist right away. (you'd need to allow anonymous saves though or keep the state for after the registering somehow.
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The spinning image makes me think that each playlist is just one track. The player controls above the list don't help either, they should be in the right sidebar too.
There is also no visual distinction between folders and playlists.
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I didn't make an account yet, but you said you can set in and out-points per video? Maybe it would be easy for you to build a good mashup player (if buffering plays along) one should be able to mute on or use sound of one and video of the other. It's super fun and might be something that people are likely to share.