Minecraft wiki entry for Development_versions, associated text: Improved NBT parsing in commands.
Keys can now (optionally) be quoted, e.g: "Motion":[0.0,0.0,0.0]
Unquoted string values are now stricter; they can no longer contain spaces or special symbols, e.g: CustomName:Test Test
Indices, which were previously optional in lists, are no longer allowed, e.g: Motion:[0:7.3,1:1.0,2:3.1]
Empty keys and values are no longer allowed.
Improved error messaging; now points to where exactly the error occurred.