Well since I've just started using my One S today, which doesn't have a menu button and thus requires all apps to put a menu button somewhere on the screen, I feel like I've gotten a little taste of what iPhone users get.
Gotta say, it's really no different. I sometimes find myself reaching for a menu button that isn't there, but I've already pretty much gotten used to it.
Any app that isn't ICS-optimized (without an on-screen menu button) gets a virtual menu button added along the bottom edge of the screen (or right edge if you're in landscape). When I first read about this a couple days before the phone arrived, I'll admit I thought I would hate it. But it's not so bad. Most apps aren't even affected.
Thus I think the biggest disadvantage of the iPhone is simply that the screen is kinda small. And I still think I'd have a hard time getting by without third party alternative apps for the keyboard, launcher, browser, etc. I suppose the idea is that the built-in apps are very high quality so you shouldn't need a third party replacement, but I really like having a swiping keyboard and a little more customization.
Still, it's true that for the majority of people, they don't want to have to think about anything, they want their smartphone to work in a way they expect from day 1 without fiddling around. And I get that. On the other hand, this is a tech forum and when your phone is missing such features as USB mass storage, alternative keyboards, or expandable memory, it's at a disadvantage in our eyes.