HTC seems to be back to full body aluminum phones for their flagships. The HTC One S that had "mostly" a metal body, but the top and bottom were plastic, and the One X was made out of polycarbonate. Most of HTC's phones have been like that, either a mix of aluminum and plastic, or polycarbonate. But I've learned that nothing beats a full aluminum body when it comes to phones, in terms of premium and solid feel.
As an owner of a HTC Legend (which is almost all aluminum), I've remained pretty disappointed with how glass phones like iPhone 4, polycarbonate ones like One X, and the plasticky Samsung ones feels. Nothing feels as solid as a full aluminum body phone. So I'm glad to see HTC is bringing that back, and they might make most of their upcoming phones be like that.
I'm not sure how they will deal with the colors, though, as colors on aluminum phones usually scuff very easily, and that's something owners of the black iPhone 5 have learned quickly. At least HTC can make a pretty black aluminum phone with micro arc oxidation, that's not just simple coating of the device, to make it dark. But colors like red, which are rumored to come to the HTC One, might be a bit trickier to pull off.
Another issue with aluminum so far, and which is why HTC couldn't really make full aluminum bodies so far, is because the radio signals can be significantly weaker under aluminum, but they seem to have found a solution around that (probably by putting the antennas right on the edges and in the body of the phone, or something like that). HTC says they are the first company to ever make a full aluminum body phone.
They also seem to have put a lot of care into how the phone's body is designed. The engineers put a lot of effort into making this phone feel great, and I hope they continue doing that with future phones. That's how you make phones feel premium: metal body and designing all the right curves.