Fewer parts, simple design, but the number one thing in my view is that the barrel is "scalloped" out just above the feed ramp. Or, in otherwords, they are not a "fully supported chamber". In saying this I don't mean it as a negative. Fully supported chamber is an idiotic and unnecessary idea for a 9mm pistol, especially anything that is not a target pistol.
Left to right: SIG, HK USP, Beretta 92, GLOCK, Walther P99, Steyr M9A1
See how the GLOCK barrel has almost another "ramp" after the ramp.
This is a similiar type of photo. Left to right: GLOCK, HK USP, Steyr, Walther. Pretty much all barrels have some material removed in this area, but notice how it is more with the GLOCK 17 barrel in this photo:
This is another way of looking at it. Notice how much more of the case-wall is visible on the brass in the GLOCK barrel versus some of the others. Left to right: GLOCK, HK USP, Steyr, Walther.
Different calibres in GLOCK are all a little different, but I think you will find that their barrels are noticably more "scalloped" out than their competitors calibre-for-calibre.
It makes the guns super reliable. Some people say it makes GLOCK brass less suitable for reloading, especially multiple reloads. Others believe there is a connection between this and "kabooms". I don't think you are going to be seeing that in a 9mm GLOCK using factory ammo... and in the meantime the gun will feed and cycle just about anything no matter how dirty it is, and a huge part of that is that scalloped out chamber.