The ability to use double slabs in creative mode is infinitely useful. It doubles building speed when using slabs, and the normally unobtainable slab with a damage value of 6 is a block used regularly by most advanced builders I've seen.
In the snapshot, this functionality has been removed. Placing a double brick slab results in a double stone slab, and double birch slab results in oak. This is incredibly annoying to say the least.
Come to think of it, blocks like Huge Mushroom that have always ignored damage value are just as annoying. I'm sure I'm not the only one who would like to build their own giant mushrooms without having to use pistons or MCEdit. Please remedy this.