*Stained hardened clay: This will add a lot of potential for builders. I'm definitely planning to use this. I can't say dying clay makes much sense, but it's still aesthetically pleasing.
*New Lapiz Block texture: It looks absolutely awesome!
*New Charcoal texture: It really doesn't fit well in my honest opinion. I might just retexture it to be coal in my default texture pack edit.
*New Coal Block texture: It looks too much like obsidian and/or crying obsidian.
*"Made mobs spawn in groups again": I thought they already did, but I guess I was mistaken.
Donkey/Mule inventories: I don't plan on using horses and mules (Retextured horses), so this doesn't affect me. Besides, if I want a moveable inventory, I'll just carry an enderchest and silk touch pick.