A few misc things to point out.
I went through and looked to see if anything other than potions was updated to an object registry, and it turns out enchantments are as well. It should be safe to assume that Forge will do to them what it did to blocks and items in 1.7, eliminating the need for the user to mess with IDs. Other stuff, namely biomes, are still not updated in this manner. I assume they will be eventually, I just don't know yet if that'll be 1.9.
I noticed that in RegistryNamespaced that the underlyingIntegerMap object is no longer the same class. I've been hesitant to call it ObjectIntIdentityMap yet since it's so different, in case the original still exists somewhere, though it does carry out basically the same task (at like twice the size). Maps and lists replaced with arrays, maybe for better performance.
ExtendedBlockStorage has been changed. What used to just be a char array named "data" to store the blocks is now a class. ExtendedBlockStorage itself is smaller, with some of the functionality moved into that new class. If I had to guess, this might be the first step towards merging the block ID and metadata fields into one large field for storing just block states. Or it's just them abstracting more stuff for the sake of abstracting it, who knows yet.
Mojang said the structure blocks were only going to be for internal use, but they store NBT tags for stuff like "author". I think it'd be a waste if they don't eventually provide a way for people to share structures with others.
Specific to the later snapshots, 15w38a reverted and/or changed some of the Block changes that they just added in 15w37a, related to the AABB stuff. Blocks suddenly don't get their own instance of an AABB variable anymore, there's just a couple of static ones now, one of which a standard 0,0,0,1,1,1 block, the other is set to null (not sure what this one's for yet). The block subclasses allocate their own variable(s) to store their AABB when needed.