Regardless of any other issues related to the menu bar's transparency (i.e. legibility of text when using certain desktop images/colors), here's another reason why the new menu bar just doesn't make sense. Simply put, it defies "physics"! (Yes - I know there's no real physics involved in a 2D, metaphorical environment, but bear with me...) As an example, take a look at this shot of an exposéd desktop i.e. all windows pushed off the edges of the screen:
To clarify: if the desktop itself is the "bottommost" level of the environment, and application windows operate "above" the desktop (as evidenced by their casting drop shadows onto the desktop), why then do those same windows not block-out the portion of the desktop image viewable through the transparent menu bar when exposéd as above? Instead, the transparency seems to ignore what theoretically is present underneath it, and continues only to show the desktop image.
Sure, it's not a big thing (and, yes - I do need to get out more!!
), and obviously it was probably implemented in this way to prevent any text present in a window from further obscuring the text items in the menu bar. Nevertheless, it's another strike against the idea of transparency in the menu bar as far as I'm concerned; conceptually and in practice it just doesn't make sense.

To clarify: if the desktop itself is the "bottommost" level of the environment, and application windows operate "above" the desktop (as evidenced by their casting drop shadows onto the desktop), why then do those same windows not block-out the portion of the desktop image viewable through the transparent menu bar when exposéd as above? Instead, the transparency seems to ignore what theoretically is present underneath it, and continues only to show the desktop image.
Sure, it's not a big thing (and, yes - I do need to get out more!!