Unlike GPL2, the Apache license has an explicit patent grant. When I first saw this announcement, I didn't realize what effect it will have on the patents used to bring up any chance they got: they join the patents already available under various approaches (due to C# standardization or the Communit Promise). And I believe you can now avoid the goofiness of all end users explicitly needing to accept the Community Promise.

The .NET reference source is licensed in a way that Mono developers can look at it without scratching their eyes out afterwards.