It would be neat to see a difference between this and Unity's WebGL output. I'd expect this would get better performance because it's native JavaScript.

Major diff in output size. Also, the build times in unity webgl export are looong.

Yep, in Goo Create, publishing is pretty darn quick.

What does this offer that three.js doesn't already? Is it as extensible either?

Three.js is just for rendering. Goo Engine is a complete game engine with support for rendering, physics, 3d audio, scripting, state machines, timeline... And there's also Goo Create which makes it super easy even for non-coders to build a scene that can be loaded into the engine.

I'm not sure how to compare extensibility in this case, but both are indeed extensible: For Three.js you might want to add a new class, for Goo you might want to add a new type of component for your entities.

And a new landing page with some 3D goodness http://goocreate.com/

I experience an unpleasantly large amount of input lag when attempting to scroll to the bottom of the page and back up in Chromium.

Oddly enough, the lag is much worse at the bottom of the page when the 3D animation is off screen.

Firefox is a bit of the opposite. Scrolling is a slightly choppy at the top when the animation is on screen, but perfectly smooth thereafter.