I too would have "liked" to have had an SD card capability, as well as a user-accessible battery.
That said, from an engineering oriented viewpoint I'm not so sure how compatible those attributes would be with the X, given the Motomaker customizability, as well as the overall design, to include its thickness. My take is that to have custom backs on the X, you need to have a structural integrity that makes it difficult to manufacture a swappable back, especially in materials like some of the woods and leathers. If you check out the back removal section for the 1st gen Moto X on ifixit, you'll see an example of a wood back that has NO other reinforcement or backing to the wood. It's likely easier to do with some of the plastic backs, which is why the G can have swappable ones. But even then, the G doesn't have a swappable battery. And to keep the form factor and size, the battery size and design itself may not lend itself to being an easily removable slab of lithium ion power. Others will disagree, but I do not relish the thought of working for Motorola and having to deal with hundreds or thousands of broken or bent backplates in woods and leathers because of ham-handed customers who wanted to change their batteries every other day.
An SD card may be something that's easier to do, especially if it's side-loaded via the metal frame rather than a slot on an opened back. But then, I don't know how much (if any) that would compromise the antenna functions the frame fulfills, as I'm not an RF or an electrical engineer. I've also used an extra SD plugin such as the Meenova in the rare times I wanted to use more than the 32 gB space on my X, but it hasn't been often. It would be great if they offer a 64 gB option down the line, so we'll see.