Well all it is is that Mono isn't a copy of .NET. It's a independant implimentation of C# and of course it shares a lot of features, but it has all it's own system bindings and all that. There is some attempt at copying winforms and such to get binary compatability, but people don't use that for normal stuff.
As far as Linux replacing Windows goes there is less and less reasons to stick with windows-because-you-have-to every year. Novell and Microsoft are working on file format compatability with O

rg (in exchange for supporting Windows on Xen/Linux and patents). Ecplispe is well on it's way to displacing VS as 'the' enterprise IDE. So on and so forth.
Most of the new features in Vista already exist in Linux, more or less. Desktop search, integrated with email, browser history, im conversations, etc. Email client on your desktop that can tie into all sorts of different groupware stuff, including exchange. With calendering even, which is a new feature for Vista's default email client. You can mount 'virtual folders' based on beagle search results. 3D compositing desktops. Even those little gadget things you can put in the side bar have existed for a long time in Linux as 'applets'. Vector-based graphics. Userland drivers for video cards (and USB devices). Hell, even trusted platform module support...
So, of course, Vista's stuff is going to be more 'complete', well tested, and smooth at that sort of stuff then Linux is right now. But considuring this is a bunch of semi-anonymous people working independantly all over the world on the internet vs one the largest and most wealthy corporations in the world's flagship product.
Vista is the absolutely best they can do. The pinnacle of decades worth of software development and the result worth of billions of dollars spent... and it's not much better at all.
Of course MS Office is still a lot nicer the O

rg.
Now if you don't want to use Linux or don't like it or can't use it and like it or whatever.. that's fine. It's your choice. I don't have any problem with it what-so-ever. Understand completely and 150%