Metro is just a skin on a desktop PC. if the person doesn't liek the skin, they simply turn it off. For day to day stuff, it likely won't be that much different than Windows 7. But for those who want the new skin that can help do some thing quicker, it is an available option.
Except that there ISN'T a way to turn it off. I looked around and found nothing. (unless I use the registry)
The way I think it could be is this - The preview may not allow it, because that si the whole point of the preview. They wan everyone to use and try to get use to the new way of doing things. Likely in the final version you will be able to.
From the videos I saw, it just depends on what device you are using Windows 8 on. For example, on some tablets the Metro UI is going to likely be your only choice. It is no different from Android or iOS where you have a single UI for controlling what you wnat to do on the device. This will be especially true of tablets that are using ARM. For the x86 version of Windows 8 used on a laptop or desktop or certain ARM tablets, you will be able to switch between the Windows UI and the Metro UI.
So if you don't like Metro, however you can move it out your way to get to the UI you prefer then do that. What I don't understand about peopel is this, if you don't care to use the Metro UI, why are you downloading it in the first place? If you prefer the old Windows UI, then just stick to Windows 7 or whatever other version you are using. The reason for Windows 8 is specific. The Metro UI is part of the what you need to use unless you need to use ither apps.
I personally like the Metro UI on a smaller screen. On a larger screen with all those stupid color will take some getting use too. But the UI is for touch and using a mouse with it, I certainly don't like. But it does work and until I get a touch display or a tablet running the new Windows, I will not use it for now. I suggest other should do the same.