It would never work, at least not exactly. Its a political thing. Basically, who wants to give their time and energy away for free to help with something Microsoft is paid to do? M$ makes money, while EVERYTHING that is Gnu/Linux is created for someone's personal use and given away for free that others that create will be willing to share in the same way. Additionally, if you make a good program for yourself and share it for free, there is a good chance people will repay you through contributions of bug fixes, additional features, or maybe a really big thank you.
When was the last time you think anyone took the time to write Bill Gates and say "Thank you Bill for making a really great operating system for my computer". Not likely, cause why bother, you got your OS, M$ got their money, no more reason to really talk to each other.
It is really interesting the relationship Sun Microsystems has with the OSS community. They give away a produce for free, and open the source. However, if you contribute, you can only make code suggestions, that may or may not go in, and Sun keeps total control. Lots of people use open office, but there is no real Open Office developer community because you can't really ever be part of a team. They are 'basic' open source, meeting the dictionary definition. They arn't putting their effort into community building. Ubuntu is quite the irony. Sun is open source and discourages community (passively) while Cannonical makes proprietary software (you can't get the code for the forums, or even help) but these closed tools are used to build community.
M$ having a site like this is like having a poetry jam where they charge admission: If you are going to read poetry, you want to get paid. But make it free and charge for drinks, but put in some comfy chairs, people will want to share for free just to be heard.
I love ubuntu. I do some programming, but nothing cool enough worth inclusion in Ubuntu. Maybe some day. I am AMAZED how much Gnu/Linux has become simply through a desire for nerds to share and help each other out. I WANT to be more a part of such a great community, but my experience and skill only put me at, hopefully, very knowledgeable user. So, I try and do support. I came on the forums for the first time in a long time and found I knew the answers to many of the questions people were asking. So I figured 'why not?'
Seriously, I get this whole, wonderfully incredible operating system and software that does everything I need that would otherwise cost tens of thousands of dollars in proprietary software, and it is all given away in hopes that people will want to get involved.
Me helping any way I can makes me feel a part of that; it is how I can pay for my Linux just the way it was meant to be.
So this is my way of giving back. If there was a site like this for Microsoft (actually, there is, sort of. it is called Microsoft Knowledge Base) I, for one, wouldn't bother. I'd say "Already paid for it, let someone else do it". Why would I want to give my time away for free? This isn't free, I get Linux!