I've used Traceur before, how does this project compare? (With an implied "why duplicate the effort?")

Oh, duh, I found it: https://babeljs.io/docs/compare/

Supports more features than Traceur and doesn't require a runtime library. Ok, that's a clear improvement :-)

And don't underestimate the value of "readable" transpiled code. It goes a long way, even beyond debugging (and no, source maps are not a silver bullet).

As an aside, I find it interesting how wide the range of open source project web page designs is. Even for really nice widely-used projects, you see official home pages that range from a markdown file on GitHub to this, which looks like a tech startup's landing page.

Nailed it.