Here are the changes that I know of. There might be more. I don't know what differences there are between devices.
- Library settings has "Book management" settings.
- Allows to remove download books, previews or "imported" books
- And Pocket article management has been moved here.
- Includes counts of books and total sizes.
- "Pocket" settings page has been removed.
- Menu on "Reading Stats" and "Awards" now has "Dictionary".
- Disabling the info panel now works for both sleep and power off.
Fixed bugs:
- Light level not being set to 0%.
- Font changing when reading an epub on the SD card and the device has been to sleep while reading.
- In-margin page numbers displayed when reading an epub on the SD card after the device has been to sleep while reading.
- Doesn't go back to sleep after about a minute if the screen isn't touched.
The above is from my notes with the beta firmware versions and a quick check with the released firmware.