I could think of a few things they could add, and change:
1.) Bring back middle clicking to copy the block onto your hand.
2.) I realize that they didn't have items in Minecraft Classic, but having to grab a stack of arrows when I want to go shooting in Creative Mode just isn't fun. So infinite arrows.
3.) Don't have stacks show their value. Sure, stacks of blocks and items could have a value, but it's not quite the same as classic if that's shown. They're infinite anyway.
4.) Disable the crafting table, furnace, dispensers, etc. while in Creative Mode. This just ruins it because it shows your inventory.
That's what I think they could do to improve MY experience, but if people disagree with it, that's alright.