The issue is how you define build quality and what factors are important to you.
When people say the HTC One has good quality they are focusing on how it looks and feels more like a premium device externally. If you knew nothing else about the S4 and HTC One other than being able to see and hold them which one would you assume is the more expensive and premium device?
If you value how the internals are organized and/or durability (especially in terms of impact) than what you are looking for in the build of a device would be different.
I think at this point generally build quality is more directed at how premium a device feels externally (oriented to most consumers) and durability especially would be another category.
Essentially all we are arguing is looks and durability.
I honestly do feel both phones are great, they both come with cool ' gimmicks' but I don't find either gimmicks purchasing points though. I am waiting for the new phablets, particularly htc/samsung, I hope that htc doesn't cut corners on their h6 as I would love to go htc this year.