Why is it a big deal it fires from a closed bolt?
It's not that it fires from a closed bolt, it's that it fires from a locked
bolt. The Sig uses a short stroke piston operation, while just about every other sub gun in popular use is blow-back operated.
Supposedly it was designed by a former HK engineer who sought to improve upon the MP5, and it least on paper it's looking good. The monolithic upper receiver allows for plenty of rail space for accessories such as flashlights or lasers. The AR style lower receiver will be familer to those who use that platform and should provide for ergonomic operation. Sig also claims that users will be bale to change barrels lengths and stock configurations in the field, though I am not quite sure if exactly how one would go about that.
The gun seems very promising for those military and police agencies who still desire a sub gun over a short barreled AR. Only time will tell how when it comes to seeing how it actually performs though.