I have been fearing this would happen for some time now, it probably first crossed my mind when iChat came out in metal (the first app that, to me, really didn't fit the whole idea of metal) but really sunk in with safari. Although in some ways I like Safari's interface, the metal interface/theme scares me. There seem to be no real guidelines as to how a metal app should look. Compare the metal interfaces in the current apps we have now: Safari, iChat, iTunes, Quicktime, iMovie, iDVD, iPhoto. There are some glaring issues with consistency just amongst these apps, and these are made by Apple no less. For example: iTunes & iCal have 1px. thick borders separating the metal area from the window content, the rest do not (look at iChat, Safari's bookmark view, or iPhoto, and compare it to iTunes). Safari's silver-textured buttons are inconsistent with the [Aqua-style] top row buttons in iTunes & Quicktime, but do fall in line with the pasted on row of buttons at the bottom of iChat. At least to me it looks as though those buttons were hastily pasted on, with no outline or recessed border, they seem almost out of place. What about the aqua drop-downs and scrollbars? Lack of customizability within apps (ie toolbars)? inconsistent borders surrounding apps (again: iTunes vs iChat vs Safari)? Consider the repercussions of allowing third-party developers to use this completely unguided interface. It completely destroys one of the few advantages that the Mac has left: consistency between applications. Already, OS X is feeling much less refined and more of a Frankensteinian, pasted together creation, much like, dare I say, Windows. While Aqua is not without problems, it is at least consistent. Finally, with metal, much of what has drawn so-called switchers to Apple (including me) in these later years will be lost: Who would really switch platforms for such a boring interface? No more drag and drop/toolbar customization, nicely-rendered photo-realistic toolbar icons, title-bar transparency, and the simple elegance (even if a bit gooey) of the Aqua interface.
I know some hate Aqua, but metal is not the answer.