I'm confused as to what I'm doing. I search for music, I add songs to my playlist, and then... what?
In which case, very impressive. I'm particularly impressed that it even finds mashups, and the interface (apart from the difficulty I had with getting it to play) is very nice.
I hope you've thought through the legal issues. It could really use a FAQ with answers to questions like "So where the hell is all this music coming from?"
I've found a couple of tracks where the time (in green on the right as it plays) was given as "NaN:NaN". That's probably not a desired behaviour. (fwiw the track was "Thunderbirds Are Coming Out" by TISM)
Other than that, I love it.
edit: I'm not against requiring Javascript for those parts of your website that absolutely, positively require it, but everything that can work without JS should gracefully degrade.
What am I not getting?
I think it's better to express as much as you can in a more "constrained" language like HTML or CSS because it's more like data than code, so it offers less opportunities to make a big mess of spaghetti code and having obscure interactions in the code. It's easier to reason about. HTML and CSS, being declarative, are much easier and cleaner to deal with than JS (it's more WYSIWYG).
So I guess what I'm saying is, coding in a style that avoids JS as much as possible is beneficial, and that style facilitates graceful degradation. Conversely, keeping an eye on graceful degradation will facilitate a better style.
I'm partly kidding. I might use that design style under some circumstances. I disagree that it's the right way to do things. There are many ways to do things. I think people want a "right" way because they want to feel like they've got stuff figured out. (This is especially common in our world because software is complex and there's a lot of uncertainty around how to do it well.)
As it happens, the app I'm currently working on is as far away from this design style as you can get. We do almost everything in JS, even things that could easily be done in HTML and CSS. Our way of carving things up relies heavily on expressing everything in a single language, and the Turing-completeness of said language is an advantage not a disadvantage. It's a solution I'm fond of because it neatly combines high-level abstraction at the source level with some stringent performance requirements in the browser. It will also be fun to watch the purists turn green when they see it.
Edit: we do almost everything in generated JS. Might be relevant.