Also, Sublime seems to be abandoned for a while. Not that it stopped working or anything.. But I believe the lack of development will push people away eventually.
Well, Emacs Lisp might be less newcomer friendly and it's not a webapp. But still, thriving ecosystem - check, accessible to developers - check, huge number of packages - check, runs everywhere - check.
I'm primarily a vim user and I still prefer vim to Atom, but within a day or two of seriously trying out Atom, I was probably up to speed with 90% of the core functionality I use in vim on a day to day basis. That last 10% is a bit of a killer, though. :)
Of course, Atom is open source and has potentially huge community backing to push it forward (vs. pleas for features and fixes to the other editor's seemingly absent overlord).
When I switched laptops recently, I made the conscious decision to only install Atom to see how things went...so far so good, although at times it does bog down pretty noticeably, and this is by far my biggest complaint. There's also no alt-drag support (to create a straight line of cursors like Sublime), but that's a fairly small gripe.
I loved the plugin ecosystem for Sublime, and in the short time Atom's been around, the ecosystem has already exploded. Lots of awesome themes and plugins are already available, with more coming every day. The low barrier to entry (most things can be done with JS / CSS) probably has a lot to do with that.