Support for epubs and RSS, much more responsive UI, night mode, Calibre integration + SSH server (you can send books over Wi-Fi), countless settings to adjust pretty much anything (for example, you can set margins to zero — the minimum margins on stock firmware are insane). The list goes on.
edit: found this link. I don't think this list is complete.
edit: found this link. I don't think this list is complete.
https://koreader.rocks/user_guide/#whatcanyoudo