Looking at the newly released ImmutableCollection package, I see some confusing restrictions in it's licensing agreement (especially for something that must be redistributed with your application)
You may not ... work around any technical limitations in the software;
Does this mean it's illegal to use reflection to get at private bits of the library?
You may not ... use the software for commercial software hosting services.
This sounds quite scary. Does this mean I couldn't make a commercial website or SAS product using this library?
You may not ... modify or distribute the source code of any Distributable Code so that any part of it becomes subject to an Excluded License. An Excluded License is one that requires, as a condition of use, modification or distribution, that the code be disclosed or distributed in source code form; or others have the right to modify it.
Does this mean I can't use GPL licensed code with this library?
You may not ... distribute Distributable Code to run on a platform other than the Windows platform;
This looks rather obvious, but does this mean I couldn't make a website that ran on Mono and used this library, or does it only mean that I couldn't make something using Mono and then package the program and this library in a Linux debian package or something else? (ie, distribute, not just run)
This license worries me greatly about my ability to use this (much anticipated) library. These questions probably require a lawyer to fully resolve, and I'm not going to buy the time to ask one.. so, I just won't be using this library, as cool as it looks and as much as this has been anticipated for me.
Unless you have a legal team you can ask these questions, I wouldn't use this library either until Microsoft changes the license to something less hostile to developers
Also, if you want a version of this with a sane license, see the up and coming MIT-Licensed Project