well the filler door has a sealed piece on it that keeps water and snow out, from what I've seen so far it actually works, I was a bit worried about that myself. If you are really concerned you can buy a keyed locking cap for the filler that would also keep out the elements, it looks very different than a typical cap, so it is unique to this vehicle and available from Ford. So far I like the capless design, one less step to fiddle with and it appears to keep unwanted water and dirt out with the rubber seal setup. I've driven through hard rain and snow conditions and never seen any moisture inside the rubber seal yet. Perhaps it might be more prone to seepage as the seal gets older and more brittle, brand new it is very pliable so it seals pretty well. | 
