I'd say the rise of air and space power projection. Those are completely new forms of weapon systems with nothing to compare in preceding history.
Also revolutions in command and control, caused by telephone, radio, computer networking are things that have no precedent in history, they are changing the way commanders can operate ('n troops, by the way), and the speed in which they can do it. This is, in a way, a weapon all of its own.
And yeah, nukes have to be up there somewhere. MAD created a whole new ball game for all nations that could participate in it, while leaving those outside the MAD lines of power open to conventional destruction.
A strange trend is that certain types of next generational weapons are getting increasingly expensive and complicated to make. Many countries that for instance in ww2 had many aircraft industries, now have difficulty getting a single multi-national aircraft out. Even america's JSF is in fact a muti-nation effort.
Oh the joys of the simple, cheap, effective AK-47 that we can hand out to criminals and children to shoot each other up and terrorize civilians with. They are indeed a weapon that is changing the face of warfare in the under-developed nations of the world. The wild west, with machine guns this time and all the chaos and destabilization that comes with it. Waging war was never easier.