I think hosting cost is the biggest issue, especially if you reach a point where you need to balance load across many servers. There is also the cost ofthe development tools, although I think MS has some programs that allow you to get Visual Studio .NET for free or for minimal cost. Other than that, I think there is just a huge bias against the MS stack in the "hacker" (yes, ironic quotes) community. Having worked with asp.net, django/python and rails, I honestly don't see a huge difference between the three in terms of development productivity, but there is definitely a difference in hosting and software licensing costs. Because of that, I've lately been choosing not to lock myself in to the MS stack for any projects that have the potential to require massive scaling.