Re: Build Qualities
First, it's important to distinguish between build quality and build materials. Most people conflate the two and then you get confusing arguments like the one you're trying to puzzle out. Then, within build quality, we have aesthetics, the look and feel of the device - which includes balance, proportion, etc, as well as the obvious engineering integrity of components and how they are pieced together - and does the respective placement of the components make sense, enhancing the user's ability to smoothly utilize the device.
So, if you want to talk about build quality, it's a huge design discussion that is had at a level that most users don't want to get into. If all we're concerned about is who is using the best materials, then it's simply a matter of: best at what? Durability? The way it looks? The way it feels in your hand? The most/least likely to slip out of your hand, off your counter, etc?
Most of Moto's praise in the materials/quality discussion is going to boil down to ergonomics and the way the materials feel. The Moto X isn't that unique in how it looks (it's basically a boxed out galaxy nexus or nexus 4 from the front of it), but the way it is designed makes it a pleasure to hold, when compared to other modern flagships. Conversely, the Moto Droid Ultra is essentially the same device with different buttons on it, and it doesn't feel nearly as good. Soft curves, great balance, the soft-touch plastic and tiny bezels that make 1 handed use a breeze are the design characteristics of the Moto X.
The HTC One is fairly great looking, people appreciate the material choice for it's perceived sturdiness, attractiveness and some other great ideas, such as slimmed bezels and front facing speakers, however other decisions, such as the placement and quality of the power button (atop so it can hide an IR blaster) created many frustrated would be buyers. "Loose" buttons seem to be a major turnoff as well. Still, the build materials choice, different design direction & a compelling update to Sense made it the only realistic challenger to the Nexus 4 for many a nerd until late Summer.
Samsung doesn't get enough credit for what they do in the materials and engineering categories, however what they do isn't evolving as quickly or drastically as other OEM's and so the response of "yawn" get translated sometimes as "eww". Touchwiz and the buttons are my biggest complaints against Samsung, however in the materials category, shiny plastic is not as "attractive" to most users, despite the fact that the plastic they use is pretty high quality in the durability category. The two biggest build quality complaints against Samsung, as far as I can tell from the forums, are the fact that the design does very little to protect the screen and charging ports that are very loose and prone to requiring replacement. As these are both things one wouldn't notice in the store, it's frustrating to purchase the device and to be required to return it due to mission critical things that don't work.
Overall, plastic is a fine materials choice and of course we're going to see it's use continue so long as it's one of the cheapest, easiest to manipulate and more durable materials available. The trick to nerd acceptance seems to be a combination of shaping, texture and appearance. My conjecture is that if Samsung's next Galaxy was the soft-touch we've experienced with the Moto X and Nexus lines, a lot of the "cheap plastic" argument goes away. Then we can put pressure on them to fix the things that matter, that horrible UI and the ridiculous buttons on the front.