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.

https://koreader.rocks/user_guide/#whatcanyoudo

Proper text justification (hyphenations + no ragged edges), auto toggle between day mode (black text on white page) and night mode (white text on black page) at sunset (via your location), compact progress display, customizable taps any gestures… it takes a little it to configure but it’s worthwhile.