Comment by dsr_

The essential bits for me:

- OPDS support including search, so I can grab books from my library with a minimum of fuss.

- page sync support, with the ability to run my own server, so I can switch devices easily. (Phone, 2 tablets, desktop...)

- page display customization ranging from the simple (margins, sizes, fonts) to the detailed (complete custom CSS, if you like)

If I can't quickly set a reader to Palatino or a close relative, left-justified, line height of 1.2, it's a failure for me. KOReader rarely disappoints.