First-time shooters want "a .40" because that's what their friend told them they needed, based on something they heard from a guy who once knew a cop. There are usually mentions of stopping power and/or knockdown power.
Way too broad of a statement. I picked my FNP40 because it felt the most natural in my hand out of probably 20 handguns. I spent plenty of time reading on several forums before I settled on 40 caliber, and picked up snap caps way early and used the hell out of them after I bought my gun. Once I was able to focus on the front sight my groups went from stupid to very good in just a couple of range trips. I understood the front sight concept but my eye/brain coordination needed some repetition. I did find myself anticipating recoil but trained myself out of it very quickly. I shoot my buddy's .45 way better than he shoots it, and my other buddy with a 357 can hardly hit a paper plate @ 7yds while I'm poking holes in a 2" center and have shot those calibers only those times. Its not prodigious talent, but it really isn't that big a deal for a beginner to shoot a .40.