What happens if full collective pitch is suddenly commanded on most modern helicopters?
I'll assume you're talking about typical turbine helicopters.  The fuel controller on a helicopter's turboshaft is used to regulate rotor RPM via its gas generator's (the compressor/burner/turbine section) speed and torque.  The RPM governor setting is coarsely set with throttle, then fine tuned with a trimming function controlled by a toggle (blip) switch on the collective to set the precise rotor RPM the governor will maintain.

The fuel controller will now govern rotor RPM automatically at that setting as you load the rotor with collective, by increasing fuel flow to increase the torque output of the gas generator.  So torque, fuel flow and Turbine Outlet Temperature (TOT) will all rise as you pull collective.  Both torque and TOT have operating maximum limits, so how much collective you can pull is therefore limited by a torque and/or TOT red line.  

Trying to take off in marginal conditions due to load, temperature or altitude, you will find yourself pulling pitch until you are at a TOT or torque limit before you can even hover, and if you really really had to get airborne (like say you were an overloaded medivac Huey in Vietnam), you'd have to get the machine sliding along the ground first to get some translational lift in order to take off (one big advantage of wheeled landing gear).

Anyway, it's a hydromechanical fuel controller/speed governor that just does what it is told, so if you just haul the collective all the way up just sitting there, it'll depend on how much fuel the controller has to dump in to maintain RPM as you shoot off the ground straight into the sky.  You will probably exceed the TOT or torque limit or both, making it a very expensive takeoff.