There are some core qualities such as reliability, size, safety features, accuracy, magazine size, price, etc., that may make one gun objectively better than -- or at least as good as --another for a specific purpose. Once you get passed that, there are a lot of more subjective criteria that can lead someone to prefer one over another. Glocks are functionally efficient and reliable, so they're always going to be in the conversation.
But some of those other qualities may really matter in terms of personal preference when deciding among the contenders. Hammer or striker fired? How is the trigger pull SA/DA in terms of poundage, steadiness, and reset distance? Does it have a grip safety, manual safety, trigger safety, or what? How about controls like the slide and magazine release? Are they easy to locate and depress? Do the mags fall out on their own when released, or do you have to remove them manually? And there's obviously ergonomics -- how it feels in your hands, whether your fingers fall in the right places, etc.. And for lefties, there's always the question of ambidextrous controls.
And there's also just looks. I was cleaning my PPK and my wife walked by and said "that's a nice-looking one". I actually prefer the Bersa Thunder in terms of usage, but the PPK is just sexy.
ETA: Forgot some shit. I have a Kimber Micro 9, which is a really nice pistol except the damn thing requires a tool to break down, and I hate that. So ease of disassembly may matter. There's also how easy it is to rack. I had a Beretta 85 Cheetah that was direct blowback and an absolute bitch to rack. The PPK isn't easy either. On the flip side, I got Mrs. Tip a S&W Shield EZ that can be racked with a pinkie. That can be huge for some folks who don't have a lot of grip strength. Some guns are really snappy as well, due to weight/caliber issues as well as how high the bore axis is. Those guns may just be not much fun to shoot because of that.