One thing a pilot can never get enough of is speed. With airplanes and rockets, you can get about as much speed as you want...as long as your bank account is fat enough. With helicopters though, there has been an upper limit...until now.
Retreating rotor stall happens at the point where the vehicle is moving forward faster than the rotor is spinning. It causes all kinds of problems and has limited the top speed of helicopters for years. (See DynamicFlight.com for a good discussion of the phenomenon.)
Several companies have been working on ways to overcome this limitation and get faster helicopters. The first one to hit full production was Boeing's V-22 Osprey. It rotates the rotors so that they'll act more like propellers. It's a decent solution, but it involves a lot of moving parts...including rotating a pair of huge turboprop engines 90+ degrees. Other companies have other ideas.