And well... it isn't. The biggest thing I can tell is that it's "hip". Meanwhile Sublime Text spins around it at 40MPH doing donuts.
I tried. I really tried to use Atom. But the text editor is just so slow and unresponsive, and that is a flat out dealbreaker for a text editor. Its extensibility isn't any better than Sublime's, and I feel like Python's a way better language than Javascript, so I'm happier tooling against Sublime.
I too wish Sublime Text was open source, coming from an open source development background, but I understand exactly how hard it is to develop open source software and feed and clothe yourself - people only buy "support contracts" for the first year and rarely renew, and only for software they feel is technically complex enough that they might actually need help or customizations.
For the record, I also backed GNOME Builder (which I also already find superior to Atom) in the hopes of some day it becoming the text editor and development environment I lust after. But my greatest fear is that more time will be spent on the shiny features and not enough time on the things I really care about like a decent GUI debugger so I don't have to live with poking at GDB, ahead-of-build compiler warnings/errors, and code refactoring tools.