There were a few rumors long before The Avengers hit theaters that there was more to Coulson than what meets the eye. Back in the days when reports had the Skrulls and Kree in The Avengers, some unsubstantiated whispers had Coulson possibly being an alien who worked for SHIELD as a liaison between Earth and whatever aliens are out there that we're going to meet in Guardians of the Galaxy.
We even asked if Couslon was not exactly human to a Marvel insider long ago who was taken aback and asked us where we heard that...
Mar-Vell was sent to Earth to spy on humans for the Kree and while this theory is a stretch upon a stretch, Coulson being an alien agent does explain his overly calm demeanor and kick-ass abilities from Marvel One-Shot titled A Funny Thing Happened on the Way to Thor's Hammer.
Verdict: There's definitely something different about Coulson.