Let's see.... A swashplate assemblly, Ball to servoarm linkages, Lots of wire-ties,,, (source, maybe a crashed Collective Pitch RC Hellicopter), which is a good thing, cause you'll also need the Helli transmitter and reciever (to get the mix logic), a BEC to power the reciever,,, Maybe a second person to pilot the quad, But I really wouldn't worry too much about that part, as soon as you try to lift off the quad, the COG imballance will give you a nice, heafty, positive pitch, which will nose dive the quad, likely smashing the camera and quad to bits. But it looks like it would be really funny (as long as it is not your quad and camera).
Unless you are wanting to be able to roll your camera 30 degrees or so right or left of horizontal, you would be better off with a simple X-Y gimble friven by two servos. But, there is a reason that the camera rigs you normally see on Helli's and Quad's are located under the center of (or close to the center of) the aircraft. It's trelated to things COG, COM, MOI, & MOM (ie: Ballance and controlability)