HTC One M9 vs HTC One M8: What's the difference?

Image may be NSFW.
Clik here to view.

HTC has announced a new flagship handset, the HTC One M9. At first glance it looks to be very similar to the M8, the phone it replaces. So is this more of the same, or is there more to the HTC One M9 than meets the eye?  We've drilled though the details...