6.3 was pushed out to my Kindle today as well.
there's now a parental controls on/off toggle option in the 'more' menu, not sure what it might control yet (would be nice if it protects from purchasing apps w/o a password). *update*: parental control restricts wifi use (going online requires the parental control set 4+ character password). however, there's no options to lock the wifi immediately after screen lock, or a certain timeout, etc. appears you need to manually disable the wifi after you use it to re-enable parental control.. also doesn't require password for downloading/purchasing from amazon app store (would've be nice).
Also seeing that the screen can now be dimmed lower (i noticed that first, since i keep the brightness at lowest setting, after the update it stayed at lowest setting, but was noticably dimmer. I also compared it visually w/another kindle i have that's still at 6.2.2 while both are at their lowest brightness setting)
I'm noticing pulse reader has been updated as well (from 2.4.3 to 2.7.6).