If you're developing in .NET, you might as well use as much of the Microsoft stack as possible. The benefit to using Microsoft stuff is the vertical consolidation and tight integration.
Otherwise, you'll be fighting against the grain. And the support community will be much smaller (i.e. .NET w/ OSS databases).
No I don't agree at all. It's trivial to connect your .net applications to any database you want. If SQL Server is $7000 too expensive it's a mistake to assume you'll be better off dumping the entire .net framework just because .net + SQL Server is more popular than .net + (arbitrary DB)
I agree about using the rest of the MS stack with .net but the DB component specifically is highly modular. Let's say you want to go for PostGre or Couch. Neither of those are part of the 'RoR stack' is it? How is is any less correct to use PostGre + .net rather than PostGre + RoR? Is the PostGre + .net community really that much smaller than the PostGre + RoR? I seriously doubt it.
Drivers dude. The drivers are SUCK for using MySQL and PostGreSQL. Couch is fine and so is Mongo but the drivers aren't baked (I worked on one for Mongo - built the initial LINQ provider).
Run MySQL on Windows and see how slow it is. The driver story is horrific.
I can vouch for the Postgres part of that. We use .NET against PostgreSQL at work (a not especially database-intensive app, fortunately) and wound up using the ODBC drivers. We experimented with Npgsql, but ran into some flakiness that scared us away. There's really not a tremendous amount of documentation around using .NET with Postgres, either, which is unfortunate.
I didn't say anything about connecting to other databases. Yes, .NET does that. But when you have more advanced problems, you're less likely to find other people who had the same problems (and then solved them) than if you stuck with Sql Server.
My opinion comes from first-hand experience developing for a .NET-based startup. I have faced the issues that the two TekPub guys have. Although, switching away from .NET isn't an option for me. So, sometimes I need to integrate with OSS components. And the .NET examples/docs/community are often sorely lacking, and I have to figure out a lot on my own. I realized that it would be a lot easier to integrate if my employer was a PHP/Ruby/Python/Java shop.
Since there's SO many people using .net I really have never had any of these problems you describe. Every database under the sun has loads of users with .net frontends - even compared to RoR users. Yes it's true that the .net+SQLServer userbase is larger than the .net+OtherDB but that does not imply that RoR+OtherDB is always bigger than the .net+OtherDB group.
Could you give me an example of the sorts of issues you've had? I've been using .net since 1.0 and we integrate with dozens of thirds party systems every year and database tools for .net are in my experience mature enough to not be an issue.
Otherwise, you'll be fighting against the grain. And the support community will be much smaller (i.e. .NET w/ OSS databases).