Appearance (with the functionality) - the HTC One sold a ton of phones, the HTC 8X, not so much. Difference ... the One had the goods, the 8X not so much (both good looking devices).
Appearance goes way beyond the hardware itself. My Start screen is gorgeous (as well as some other 8.1 pix on this forum). One major complaint about the Start Screen is it was too busy all those flipping / updating tiles in basic monotone colors. It ran it's course & quite frankly I can't believe how ugly that was. With Dev Preview, you have transparent tiles (if you choose) with a static background - looks better, your favorite pic / scene / abstract.
As far as plastic, there is plastic, then there is polycarbonate - both plastic, yet light years different. My Lumia 920 was a tank, the Lumia 1520 I currently use is light for it's size, however very solid. I really can't believe how many devices Samsung has unloaded, good innards, less desirable fa?ade. The removable battery was one differentiator. The HTC One was at least as good as the SGS4, my opinion & critics choice, but ... Marketing must be getting huge bonuses.