Besides the water proofing what makes the Z so special?
The S4 has a better screen (more vibrant, better viewing angles, etc), much much better battery life, better RF performance, removable battery, larger dev community, louder speakers, and to top it off, comes with USA LTE bands for AT&T.
The HTC One? build quality, screen, boom sound, etc.
Not to mention, the Z is hampered with an outdated Snapdragon processor, graphics chip, etc. Doesn't hold a candle to the S4/One performance wise either.
I wanted to like it (I started this thread BTW), but it's a huge let down