My understanding is a gyrocopter has a fixed pitch on the blades I'm guessing as I don't know much about helicopters is auto rotation is fairly uncontrolled and as you say a pretty steep decent!
I'm Sure rotorheadcase or one of the other sky jockeys can expand our understanding....
Chucks in with (possibly wrong) explanation:
Both machines generate lift by the blades being moved through the air (durr).
The autogyro must have air passing upwards through the disc (that' why it's at a slightly odd angle), whereupon the blades change the angle of the air to provide lift and rotation to the rotor. The forward thrust is by another engine. Pitch and be variable on an autgyro rotor.
In a helicopter, in normal flight, the rotor blades generate lift by forcing air down by using energy from the main engine. Forward thrust is provided (usually) by tilting the rotor disc forward to change the direction of 'down' to have backwards component.
In helicopter autorotation, the disc must have air passing *up* as in the autgyro. Pitch is normally minimised and the energy to drive forward is converted from gravitational potential energy (height) and whatever forward motion there is left, as helicopters don't often have another method of forward thrust.
I think 'uncontrolled' may be a bit harsh, but I understand there is a limited range of control, and you only get one chance
Far better explanations:
http://en.wikipedia.org/wiki/Autogyro
http://en.wikipedia.org/wiki/Autorotation_(helicopter)
C