Regarding requirements, we were looking for something that was free and open source because we are planning to scale up our operations in the near future. Because we have both Windows and Linux servers, we also wanted something that could run on both platforms. Unfortunately it turned out that MongoDB doesn't perform well on Windows, but you can still use Windows servers as replication targets which is great.

Given our C# code base, the fact that MongoDB has a couple of somewhat mature C# drivers was definitely a factor.