010101 new admin UI, a shed load of deprecated/removed events, composer meta data, removal of some BC layers, etc...
It's likely that the core team will publish an upgrade guide when beta 15 is released, which will details all of these changes.
As an example, I quite often see in extend.php the listening to the serializing event. That's now deprecated, to be removed in beta 16. Best to replace anything deprecated ASAP 😉
The majority of FoF extensions have now been updated, so a good place to start would be to look at the changes required for those extensions, I would feel.
