Yes, the slide is steel. It wouldn't be much of a gun if it weren't. And those internal parts are a lot tougher than they look. I have no idea what the alloy is or how it's heat treated, but they're pretty much indestructible. A lot of people modify their competition Glocks by drilling a hole in the trigger bar, for a different pull. It's very common to read on Glocktalk about people wearing out a half-dozen carbide or titanium drill bits before finally making the hole, hours and hours later. And that trigger bar is barely over 1/16" thick! It really is obscenely tough metal.
Cost and simplicity. It's designed so that you never have to deal with a loose spring when field stripping the gun, or worry about a guide rod breaking a window. And when the spring wears out, you throw out the entire thing, guide rod and all. Apparently the flat spring is quite good, because Glock is the only brand I know of where people recommend the factory springs over Wolff springs. Glocks may not be the prettiest guns, but the company definitely got more things right out of the box than any other gun company in business today.
The heavy slide helps with the recoil. Instead of a relatively light, fast-moving slide, you have this gigantic piece of metal getting pushed back at a (relatively) leisurely velocity. Also, the bore axis is fairly low. And despite the almost universal hatred of the Glock grip angle, I find that it allows a stronger wrist lock, which also helps with recoil control.
You pretty much hit the nail on the head. There's nothing you can really do to "improve" a Glock, except for the feel. Most "improvements" center around the trigger pull. Wolff increased power striker spring, NY-1 trigger spring housing without the spring, and a 3.5# connector = a smooth, 8 pound trigger with minimal stacking. Everyone that's handled my G23 has loved the trigger pull. "Wow, it's just like a revolver!" You can also have the grip trimmed a bit if it's not comfortable. There's plastic to spare.
They work fine without any lube, too. I kind of found that out accidentally. About 150 rounds bone-dry, before I realized that the gun wasn't greased at the factory, and the previous owner had run a bunch of rounds through it without oil.
The magazines are another high point of Glocks. Like the recoil springs, Glock mags are one of the few brands where people say to forget Wolff replacements.
You also forgot the Tennifer finish. It may be ugly and plasticky looking, but the Tennifer finish is just about the strongest, most rust-resistant finish out there.
Goes bang every time and puts bullets exactly where I point
And my Sig has been my daily IWB Concealed Carry pistol (and love it) but I decided to get something else to replace it since the Sig I own is a rare bar coded ICE/DHS T&E pistol. I have heard great things about the 10mm but the platforms that support it are rare. The best (and damn near only) concealable 10mm I could find was a Glock 29 which I bought and now have to learn to like. One of the selling points I considered was Glock's interchangable magazines. I can also appreciate the extreme durability of Glock but for accuracy shooting I still think the 1911 wins hands down.
