In addition to stuff mentioned by others, things I like:

- Fully customizable status bars, top and bottom (I like book progress at the top, chapter progress at the bottom, various indicators like remaining time, brightness level...)

- Custom dictionaries (like reader.dict, very complete), and choice over how to display them

- Advanced typography stuff (override publisher font if you like, add all the ligatures...)

- Fully customizable gestures (jump anywhere, recent locations, skim book, bookmark page, adjust brightness...)

- Book map: see a schematic view of the whole book, with chapters / subchapters length (gives me a feeling of how much I've read and is left to read, as would a real book)

- Page number / count that "just works" (epubs on kobo don't count actual page flips which I find annoying)

I’ve only just installed it this week, but for me:

- DRM support (via a Plugin)

- Dark mode on an older device

- SSH Server

- Custom dictionaries

Next on my list is to find a plugin that can transform imperial to metric for when I’m reading American books

Edit: oh and also something that syncs my progress on books not bought from Amazon on multiple devices

    The woods are lovely, dark and deep,
    But I have promises to keep,
    And kilometers to go before I sleep,
    And kilometers to go before I sleep.

Auto dark mode after dusk based on your location

Better page flip area options

Active corners that can do custom things and with layers

More text layout options (although the stock kobo firmware is decent at that already)

It’s a bit cumbersome to setup but once you’re done it pays back in how pleasant it makes the device to use.

It's very configurable and extensible. As some have mentioned, the interface can be a bit confusing at times but once you're used to it it's mostly out of your way. I like that I can run it on both my Kobo and android phone and sync progress. I also self host an opds server but you can also connect to calibre, syncthing, or cloud storage to pull down books. There's even a plugin to sync my progress to hardcover, which is a more open alternative to Goodreads.

A few things I like:

- More frontlight options. Ability to toggle on/off easily using corner taps,

- More page layout and typography options e.g. custom fonts,

- Ability to load from Calibre over WiFi (I never plug mine in via USB).

Worth nothing that KOReader doesn't replace the default Kobo firmware. It's easy to install and you launch it from the Kobo firmware so it's easy to quit and go back to normal.

The overwhelming thing I hear about KOreader is once people use it for their way of reading (including doing things they coudlnt) they can't use an ereader any other way again.

You can read epubs directly. Cloud storage. It's got like everything so you should check out the features list.