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Does everything you do have to be full screen?No. You can have two apps open side by side.
>Is having multiple windows open and multi-tasking a thing of the past?
According to the tablet market leaders (Apple and Google), yes. BTW the desktop still supports this for power users. Other comments have pointed out that non-power users likely won't care about this issue.
>I know I'm coming from a place of ignorance... I'm sure I don't quite understand how to use the OS... I design software for a living.
And herein lies the bigger issue. You're used to the old way. You're comfortable with the old way. You like the old way. So do I. That's not a bad thing, and Microsoft left in the desktop for people like you and me (and most others on this forum). Nothing has changed there. I run Windows 8 on my laptop and spend 99% of my time on the desktop like it's Windows 7 with some nice new features added on (and a better security model).
However, a great number of computer users seem to prefer the simplistic style of iOS or Android. It lets them focus on one task using a very simple menu to get from one app to another. They don't need multiple windows overlapping each other, and they don't need tiling window managers. They don't want multiple open windows distracting them. This is why the iPad was made, this is why Android tablets are made, and this is why Windows 8 and Windows RT were made.
For us, we'll complain about the limitations of the Metro UI then switch to the desktop and forget about it. For them, they'll stick to the RT components, possibly momentarily confused by the drastic change from Windows 7 to Windows 8, then settle in and forget they're using a PC. That's probably a good thing. The last thing these users need is their personal IT guy going on and on about how Windows 8 is awful and the RT components are awful and it shouldn't be used. How long have we all spent trying to teach our parents/grandparents how to get around in Outlook and Firefox, writing down a list of steps they'll need to take to get to their email, or how to bring up the new picture of their grandkids? I actually bought my grandma an iPad so I wouldn't need to do remote support over the phone anymore. 5 minutes of instruction and she was ready to go.
I think RT will be a good thing for your average user. I think it will be a mild grumbling point for power users, deserving no where near the current amount of vitriol spewed on the Internet. Think of the RT stuff as an iPad built into (and complimenting rather than replacing) your desktop. You don't have to use it.