First of all by virtue of the source being open it means that there are now more eyeballs looking at it and that it is likely that they will catch potential security issues that otherwise might not have been noticed under the limitations of even the largest of corporate teams. This is one of the benefits that if it works, you'll never really "feel", since it is preventing something bad from happening rather than making your day to day development easier.
Microsoft has also stated that they are accepting contributions from the community as a whole. This is a huge change both for them and I suspect for the .Net / Microsoft development space as a whole, since it will give the community a much stronger voice in the direction of those projects that are open-source and open to contribution.
Given the rhetoric out of Microsoft and what we saw at the Build Tour New York (sorry if we missed you by the way), it seems that Microsoft intends to keep moving in this very positive direction and will continue to expand its community involvement.