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.
Some notes:
Almost all int x, int y, int z parameters have been changed to BlockPos class
ForgeDirection has been removed, replaced by net.minecraft.util.EnumFacing.
All FML classes have moved from packet cpw.mods.fml to net.minecraftforge.fml
Fluid Rendering has been disabled for the time being, to be re-evaulated and a test mod created for it.
Minecraft now uses a Model based system for rendering blocks and Items. The intention is to expand the model format to better suit modder's needed once it is evaulated.
As such, The model loaders from Forge have been removed, to be replaced by expanding vanilla's model format.
Metadata has been extracted out in Minecraft to IBlockState, which holds a list of properties instead of magic number metadata. DO NOT listen to the fearmongering, you can do EVERYTHING with block states you could previously with metadata.
Stencil Bits are disabled entirely by for the main Display, Modders must enable and recreate the FrameBuffer if they wish to use Stencil Bits.
LexManos