When I have several Firefox windows open and want to access one, there is no way to go directly to it.
Fair enough. Personally, I like that behaviour (I have so many windows open all the time that I'd never have any space for a description left in a taskbar if I had one (hell, I often have so many PDFs open at once that even when they do their expose-thing I can't find the window I want ...) But I can see why it would irritate. FWIW, I believe the latest incarnation of Unity allows you to scroll on the dock icon to cycle through the windows of that app (but since I haven't upgraded from 12.10, I can't verify this ...)
Hell, want to launch more than one instance of the Calculator? It cannot be done.
Actually, you can do this one -- just middle-click on a dock icon to launch a new instance (the middle-click-opens-new-window paradigm is standard in many apps, so it shouldn't be a surprise). It's a minor point, though.
I guess I'm lucky/unlucky in that Unity fits my brain absolutely perfectly -- it's by far my favourite WM ever (and I've used pretty much every WM that was ever written for X over the years). Both the dock and the dash make my life so much faster, it's not funny. But I agree it's not a very customisable WM (and I'm not sure why either -- Shuttleworth and his design-gurus are clearly very particular!)