I don't know what kind of usability impediment you're talking about? To be honest, I'm kinda tired of hearing this, because it seems to come from people who never really used Unity. I have extensively used KDE (v 2), Gnome (v 2), xfce4, Unity and now i3. I tried Unity simply because I was puzzled by the conflicting arguments. Power users complaining so much, while newbies were raving. I thought I would test drive it for a month just to see, I ended up using it for a year. Unity isn't perfect (no graphic environment is), but it does have its highlights. Most of the essential functions are documented in the cheatsheet that appears when someone keeps the WinKey pressed.
In my opinion, most power users who complain never took the 2 minutes required to understand how that desktop works and presumed that all they had to do was to plug in their mouse and click away. If they'd taken the time, there would be less complaints, since Unity is rather keyboard centric. Newbies on the other hand thought "let's learn Linux", read the manual, and came out ecstatic.