That said, if you can get the project profitable enough on your own, maybe you can afford to hire additional .NET devs.
Where are you paying .NET and Ruby developers 10x what you pay PHP folk?
If its an experiment or an exploratory project, well, use whatever appeals to you.
1- The stackoverflow engine is a great example. Also you have the power of SilverLight, if you want to make your application interactive.
2- You are not really locked in, you should just pay the server licenses. Expensive, but that's the price you pay for using MS products. Read Jeff Articles about StackOverFlow, they have mentioned licensing and its matters.
3- Not So Cool? You already said, it has an awesome IDE?
But I wouldn't pick it unless there is a large pool of .net talent you can pick into - as startups I have worked with here in the US who have been .net based have found it hard to hire top-quality folk into a .net stack.
If you are looking to explore and are open to learning further, by all means, jump into Django. On the other hand, if .NET access is no problem to you and your collaborators and that it fits your problem scope, go forth.
In general, Rails seems to be really popular. That doesn't necessarily mean it's the right tool or the best one.
Drop me a message if you would like to know more about ASP.NET in the production environment, automated build/deployment, affordable hosting options etc.