If you think back, every version of Windows since XP has allowed you to disable the new theme. With XP it was the Luna theme that you could disable and have it look like win2k or even Win95 if you wished. With Vista and 7 it is Aero you can disable to have it look like Win2k or even Win95 still if you wish.
I'm quite positive Metro will be the same. It might be Metro or the old school Win95/Win2k look but even if those are the only two options I'd still be pleased, so long as the option is there for the traditional desktop, taskbar, and start menu. Themes and such can be downloaded later. Heck, this could actually turn out to be very positive for MS- if all they incorporate is Metro or Win2k/Win95 themes, then third part developers can come up with "prettier" themes leaving MS to focus more on the functionality of the OS. I'm okay with that. I used to love messing with downloadable themes and shells for XP back in the day. Would be fun to start tinkering with that again.
Just so long as there is an option to disable metro and have a desktop, startmenu, and task bar I'll be happy.