Recs.
Updated
SpecsUpdate
Pros
Pro Extendable
Due to its modular design, almost any aspect of the editor can be changed. Even seemingly core packages like the one's taking care of search and replace functionality can be forked on GitHub, changed and replaced in the editor.
The Documentation for creating new plugins is also great and thus it's easier for developers to jump in and create plugins for Atom.
Pro Superb Markdown support packages are available
There are quite a few packages available for Markdown support in Atom. Markdown Preview Plus has real-time preview of markdown docs, with math rendering and live reloading. An even better one is Markdown Preview Enhanced, which, with minor configuration, supports PDF conversion, complete with Table of Contents.
Pro Multi-line select and edit
Multiple cursors & column selection allow versatile ways of editing.
ctrl + d
will select the current word and each time the command is repeated add the next occurence of the word to the selection.
ctrl + click
or middle-mouse click
will place another cursor in the place that's clicked. Cursors can then be controlled together. This also allows selecting vertically.