@maPINES: I personally think the ship works well because Big Mac and Fluttershy are both quiet gentle-natured characters. Also, I don't find the "They've never even interacted together" thing a legit argument because we don't know how often they've interacted off camera. That argument only really makes sense when applied to ships that involve two ponies that would have no possible way of knowing or interacting with each other.
/ship rant