It has incredible simplicity, combined with depth and power. Even if you don't know emacs in the first place, org-mode is a compelling reason to get into emacs, and you don't need to know much emacs to get around in org-mode.
I switched to using org-mode after trying various outliners and organizers and project planning tools over the last two decades. Org-mode comes closest to doing what I need to do. The integration with emacs in general is a plus, ultimately compelling me to get off of other tools like an independent chat client (dropping pidgin for erc and bitlbee), and switching to using GNUS rather than thunderbird.