Mode S in a nutshell
a) will cost a lot, especially in relative terms for "low cost" private pilots both initially and annually
b) will not reduce the risks of GA/GA midair collisions
c) will enable heavy commercial traffic to use shortcuts outside controlled airspace and save money
d) will enable unmanned aerial vehicles to be flown in class G airspace to do whatever UAV's do and make money
e) will avoid the CAA having to turn the whole of the UK into controlled airspace (according to the CAA)
There are a lot of sub-clauses to the above basics
So the GA fraternity (including microlights, gliders & hang gliders) are each being invited to cough up several grand for installation of mode S, plus several hundred per year licence & calibration costs in order to facilitate airlines and UAV operators to make more money.
You could look at the CAA website and other ModeS threads on PPrune for much greater detail