From what I'm learning about e-cigs, it seems to be the level of nic and the atomiser.
Nicotine has a numbing effect as I'm anyone will testify who's either got the juice on the lips/in their mouth or ever used nicotine gum. Makes sense that the higher the nic level the "harsher" the throat hit. My JC British Bulldog Medium gives my throat a jolly good whacking where as my Cigar Zero is smooth with hardly any hit. I'm sure the other ingredients such as flavourings may play a part too.
A good atomiser (+ decent battery charge) is obvious. You don't get enough vapour, you will not have enough substance for the throat hit.
But certainly one thing that has been overlooked is vaping technic, or rather more preciously inhalation technic. I've discovered whilst vaping the Bulldog that if I change how I inhale the vapour after the pull it can dramatically change the strength of the hit.