Yeah, there are Microsoft patent traps out there for code that uses core .NET features in idiomatic ways, and Microsoft's patent grant only covers what's necessary to implement the core CLR. They also only used to cover patents that were essential to implementing the CLR, so just using code that implemented it in the obvious/efficient way could land you in legal hot water. I'd hope they've extended it to cover their actual implementation now that it's open source.
The patent promise that the MIT-licensed CLR has been available with since .NET Core was announced last year is completely different from the patent promises MS has used in the past, and definitely applies to Covered Code (i.e., what they are shipping), just like Apache 2.0. In fact, it's almost as if they referenced http://www.fsf.org/news/2009-07-mscp-mono while crafting the new promise, to intentionally address the problems in that post.
> just using code that implemented it in the obvious/efficient way could land you in legal hot water
Apache 2.0, GPLv3, and MPL2 are no different; you can't take Covered Code that company X is distributing but that doesn't infringe their patent A, then modify it to make use of methods/techniques that do infringe patent A, and expect to be indemnified from litigation. And each of these other "sufficiently-free" licenses explicitly says so.
Having said all that, I do wish they would have expressly used Apache 2.0 instead, just for good measure and to avoid the casted aspersions.
