From a hardware perspective, there are quite a few exciting things about this One S.
Pros:
Personally, I really look forward to see the performance of the Snapdragon S4 in the One S. It is going to be a step forward of the Snapdragon family and its performance is quite on par with Tegra 3. If you follow tech site like Anandtech, you will get what I mean. In terms of sheer power, the S4 clearly outperforms anything out there, and probably only second to Tegra 3 with regards to graphics.
Imagesense: Camera quality is drastically improved. In the past, HTC cameras were quite bad (if not among the worst), this is indeed a step into the right direction.
Cons:
Micro-sim: This is probably a problem only for me. I have a standard SIM and I don't want to cut it.
No microSD card: I don't expect to see a lot of development for this One S (and probably the whole One X series). No SD cards means that all of the methods I know to flash custom ROMs do not work any more. My knowledge of flashing ROM maybe limited, but the fact that there is no microSD card is enough to drive most people away from the whole One series.
I may be wrong though, but compare to the Sensation, the whole One series is a major step down for developers, at the same time it is a step up for casual users.