What's less usable? I was suspicious initially but after using it for weeks usability seems about the same, if not a bit improved. It does need a lot of polish, and some of the built in apps are uglier, like clock and notes.
Transparencies make things less readable. Tap targets are less obvious. Wireframe icons are harder to parse visually. Thin fonts (thankfully larger now) are harder to read. Undemarcated areas of UI make it harder to distinguish controls from content (not a feature IMO). I actually like flat design but they've taken it too far.