Actually the galaxy s III has INCREDIBLE build quality.
Nothing has stopped working, fallen apart, gotten stuck (home or power buttons etc) or anything like that. This phone is lightweight, yet built like a tank with very good build quality.
Don't let the plastic fool you, this thing is one of the highest quality built phones out there
Yet the somewhat shiny, somewhat slippery plastic is what you notice about the S3 first. The S4 isn't much better in this area, both have a certain cheapness to them. By comparison I haven't heard anybody mention for example the material used in the Nokia Lumias so it's not like a plastic case can't be done right. It just has to look and feel good in your hand. That doesn't mean your Galaxy S xxx will fall apart, it's just about how people perceive the phone.
The iPhone is a testament to good product design. It hasn't changed much from the 4 and likewise iOS hasn't changed much either. It has not required huge changes because how it works is intuitive and easy. Most people do prefer something that doesn't get in the way and "just works". Unfortunately this is not so good for the enthusiast who wants to customize, extend and personalize. These are the same folks who find it "stale" because it isn't "the new thing", something to wow them for the short run. But it doesn't have to be, it still works fine. Likewise the hardware is solid but again won't impress those who want "the new thing".
That said, the apps running on iOS have changed. There are now a lot more apps that would be great replacements for the default apps but the functionality to make them default is not there unless you jailbreak. Same thing is happening when trying to communicate between different apps, it's just awkward. I hope Apple does something about this in iOS 7.
I'm looking to buy an Android phone for my next one as I'm tired of having to deal with jailbreaks to make my iPhone 4 work like I want. Unfortunately at the moment nobody makes the Android phone for me: smaller physical size, reasonably powerful, high res display (at least 720p), good battery life. I'm hoping the HTC M4 would be that or Google announces something at Google I/O, but we'll have to see.