The only amazing stuff is the distribution mechanism really. Other stuff is just plain old OpenGL but in your browser this time. It is not like it has some crazy support for raytraced voxels or something.
That's the main thing, but not the only thing.
WebGL is also very portable, more than other flavors of GL. Huge effort has gone into that. As a result there is a much better chance a WebGL app will give the same output on different browsers/OSes/GPUs (and if it does not, that's a bug that should be fixed).
WebGL is also more secure than OpenGL, since it was designed to run in a web browser, that executes unsafe code off the web.
Also - as I understand it, what's impressive about all this isn't just the graphics. Once you've passed vertices etc... off to the graphics pipeline then yes Javascript doesn't handle much of the load .... but as we know a lot of games are CPU bound, and the fact that Javascript is increasingly able to offer acceptable performance relative to the native compiled experience of c/c++ is pretty nuts.