I ran into the same issue last night, I just got a moto x DE for my wife, and a new laptop since last March when I unlocked my moto x DE. "adb devices" returned nothing, and I had a fresh install of the SDK and the moto drivers. Both old and new laptop on Win 7 64-bit. I googled trying to find a solution, but didn't find anything that worked. Grabbed my old laptop, hooked up to it, and it worked fine.
One suggestion I saw and didn't try was to remove/reload the moto drivers. Since I got it working and got her phone unlocked with my old laptop, I didn't pursue further.