As a veteran Stack Overflow user, and a fairly new Tridion user. I can say that Stack Overflow is great for questions of syntax and for algorithmic errors or often questions of architecture.
However, it is difficult to get answers on niche topics such as SDL Tridion, because these questions tend to get buried beneath the more common language specific questions on the popular tags such as C#, jQuery, etc.
A community specifically for SDL Tridion would lower the barrier to entry for people to share their knowledge making it far easier than it currently is to give back to the community. The Stack Exchange platform is proven, fast and mature.
The reputation scores create healthy competition between experts as we've seen on Stack Overflow - the top users have become minor celebrities on the web. It would be a good measure for MVP status.
As you say Dave, Tridion is distinct from Stack Overflow in that a lot of the questions that arise around the product are not programming related, which would likely be closed as off-topic on Stack Overflow, there are issues of content management for example and techniques such as Blue Printing and Compound Component Templates which simply do not exist in other products.
Finally, I think ideas evolve from discussion and it would ultimately become a brilliant resource that could drive best practice and the product's reputation.