Air drop is specifically a hardware feature. Your hardware has to have enough "MIMO" antennas to stay connected to the regular wi-fi network, and split one off for a dedicated channel. It's the same physical chip that lets your a AirPort Extreme run two network SSID's at the same time.
Which is wonderful and makes sense, but for me the only two AirDrop devices I have (iPad mini and Mac mini) cannot share files with each other because iOS->OS X doesn't work. While I'm not certain of why there is a limitation, I'm pretty sure it's because of the lack of support for various file types within iOS. While understandable, it is a bit obnoxious to name them the same but exclude making them compatible with each other.
Perhaps in a future update.