- Better and faster UI. I always thought the Kindle was just slow, nope, it's just really bad software
- Customise any/everything. Font size, line spacing, paragraph spacing, margins, header/footer content, the list goes on
- Ability to use things like BookOrbit (or alternatives) to manage your books and sync your progress/stats.
- LLM-integration through plugins like KOAssistant, highlight a word/phrase/passage to get it explained to you. This was invaluable for me when reading through a series from the 1950's about the late 1700's, lots of phrases I'd never heard. Many I could grasp the meaning from context but it was way more fun to hear the "why" behind the phrase. Same with historical events that I am less knowledgeable about than I wish. Let's me stay "in the book" instead of picking up my phone.
- Full control, some people don't care/need this but I love being able to SSH into my Kindle and do whatever I want. I love having my entire book reading experience end-to-end be fully open source and self-hosted.