Taking feedback from here and IRC into account, here we go again:
i like the third one, but the regions are harder to differentiate now. could you (or paul) tell us the reason he doesn’t like the regions coloured? maybe using just a slight tint on the look you did in the third mockup?
i think the header buttons look a bit amateurish (sorry, nothing personal) with the dark border around them like that. any reason we can’t just change the gtkrc file to remove the button border for the mouse-out state? ie. make them like the buttons next to the ‘slide edit’ dropdown? that should (i think) just be a one-line change in the gtk file, no need to even recompile…
the transport buttons look great, but i think they should also be flat except for on hover etc. i seem to have a thing for ‘on hover’ don’t i 
the meters i think would look better separate from the faders. i think with lots of outputs they will look really chunky. i think the faders should be the same size on all the tracks, as they are in the meter window.
keep it up, it’s looking great (despite all the negatives i seem to bring up)! 
porl
porl: Paul finds different coloured regions hard to look at.
I don’t want to make the transport buttons all flat. There should be some hint they’re clickable.
I initially suggested having the transport/etc. buttons with no “button”, i.e. just an icon… however after having used Ardour some last night I find that thorwil’s desire to keep the “button look” is well-justified: there’s immediate functionality evident when there are obvious buttons. Plus the user knows, without having to mouse-over, just how large the clickable area is. And it’s got to be easier to code.
Workflow should be the #1 concern, followed by an intuitive interface, with good looks third. So I’m going to put my vote in for practicality over appearance. Not that my vote is worth much. 