Major things to note:
Class renames: https://gist.github.com/LexManos/44dd211f90f498ad4015279b103dff86
Tile Entities are now packed in the ChunkData packet.
Forge intends to work around this to better support large moded worlds, but for the time being modders should implement the new function carefully and only send what data they need to!
Minecraft's codebase now has annotations, these are directly from Mojang and should be adheared to!
Added support for package-info.java's containing @Nullable information for all MC code base.
cpw