I'm late to the kvetching party, as I've only just installed 2013a last night. But I am compelled to echo many of the criticisms made in this thread (particularly the comments of Thomas Hoffend).
The ribbon UI style in general takes a lot of criticism, but ultimately I don't really care whether things are in drop-down menus or icon toolstrips. Unfamiliarity is fleeting; I can adapt. And in many cases (save, open, etc), sure, I'd rather click an icon than click through drop-down menus. But here is what I do care about in a MATLAB UI:
(1) Efficiency of screen real estate
I dock everything into one window (command history, current folder, command line, editor, figure -- all in separate tiles). That is by far the most effective layout for me. So space is at a premium, even on a large monitor. It is utterly unacceptable for the toolstrip to take up 1/10 of the screen. I see suggestions from MathWorks folks that it can be hidden, but hiding the toolstrip merely doubles my clicks and mouse strokes. That sounds like nitpicking unless, like me, you spend entire days/weeks/months clicking and mousing in MATLAB.
At minimum there should be an option for small tool icons, spread horizontally; I don't need the Open icon to take up 3 times more space than the Print icon. I don't need a group label telling me that New/Open/Save are "File" commands. A disk icon means "Save" in every piece of software, I don't need it to be labeled, use hover-text if you must. The toolstrip should be customizable. I have never once, in 15 years of heavy use, used New>Variable; I certainly don't need a dedicated icon for it now. You've basically locked the toolstrip into 'novice mode'. How could you not foresee experienced users being irritated by that?
(2) Efficiency of navigation
During my typical usage of MATLAB, I am alternately coding and using the debug tools in the Editor tile or generating plots and using the figure tools in the Figure tile. In doing so, there are prolonged repetitions of mouse movements between the tile and its accompanying toolbar. Relocating all toolbars away from their respective tiles results in a significant and incredibly frustrating delay. Similarly, a customizable toolbar (Quick Access) is a fine idea -- unless it's permanently anchored to the upper right corner. At minimum, the toolstrips should be dockable to their associated tiles. Each being customizable would be gravy.
Window controls (max,min,dock) are now relegated to drop down arrow icons. The tiny individual icons of the prior design took up the same real estate that the new drop down icon now inhabits. Clicks and mouse strokes to use these controls have simply been doubled.
For many many releases now, everything in the UI has been dockable and tileable and resizable however the user wishes. This continues to be a great strength of the MATLAB UI. Your implementation of the new ribbon toolstrip is completely inconsistent with that approach. It cannot be moved, it cannot be resized, it cannot be docked to the appropriate tile. The only flexibility of the new toolstrip design is the Quick Access toolstrip, which itself cannot be moved, resized, or docked.
Similarly, the benefit of an icon toolstrip -- identifiability and accessibility in a single click -- is not reflected in other UI changes. The individual sub-window controls (formerly very reminiscent of an icon toolstrip) have been relegated to drop-down menus beneath a nondescript down triangle. So, you got rid of drop-down menus in favor of icon toolstrips in one place, but replaced an existing icon toolstrip with a new drop-down menu in another place. What?
I will be reverting to 2012a, which is vastly more effective for the way I use MATLAB. Ironically, I was auto-emailed a "MathWorks Software Maintenance Service Renewal Quote" shortly after installing 2013a. Seeing that I will be rolling back to 2012a, it makes my maintenance renewal decision an easy (and cheap) one.