I guess with .Net whilst there are a lot of people using it, the culture is such that there are less people releasing stuff openly than in environments that are themselves open and thus it's an established part of the culture. Microsoft never really promotes an 'openness' and seems happy to alter the API / patterns however they like, whenever they like, whatever helps drive their own products. I think this sets a tone that ripples around the community.

Java is a funny one - whilst it was proprietary tech it still had an 'open' culture because of the vendor independence and the long-running Java Community Process (JCP) that let experienced people influence the API direction. I think that led directly or indirectly to a large number of quality open source projects, like Hibernate, Struts, Spring, Eclipse. I think this shows that even your tech doesn't need to be open so long as your process and attitude is (of course, Java is open too now), to promote the same thing out in the community.