Here is the ones that are most important for me:
About the Atom itself I love key-bindings (and the fact that they are hackable) It makes coding much easier and fun. Creating panes, change focus, folding/unfloding, select line, Select word all are possible from keyboard.
Customizing style of the page may not seems to be a big deal but is actually pretty big. I use Atom while it looks like sublime (The best of the both worlds).
There are other features like pane that is very useful once you're working with more than one file which is almost always the case for me.
I use Autocomplete Plus Python Jedi which does full auto-completion for native and custom libraries and objects. It makes everything easier.
Also mini map is very useful for larger files. It's similar to sublime's side code review panel.
Hydrogen is also a great package but won't be particularly useful for Rhino/Grasshopper development.
PS: I opened a discussion on Atom discussion page to see if there is an easy way to develop in IronPython with atom. That's what I really need. Then I can use filepath and Grasshopper component to execute the code for testing.