I've read that the .NET Core framework library (which has been open-sourced yesterday) is not a superset of the .NET 4.5.x framework. It will (only) feature around 10% of the framework library.
What are the exact differences? I couldn't find a comparison chart.
.NET Core and the .NET Framework have (for the most part) a subset-superset relationship. .NET Core is named "Core" since it contains the core features from the .NET Framework, for both the runtime and framework libraries. For example, .NET Core and the .NET Framework share the GC, the JIT and types such as String and List. We'll continue improving these components for both .NET Core and .NET Framework.
Consider the subset we have today a down-payment on what is to come. Our goal is to open source the entire .NET Core library stack by Build 2015.
But it's not meant as a total replacement of .NET 4.5 and its predecessors (yet). It will ultimately offer a lot of the same functionality as what we have now with .NET, but then entirely open source. Meanwhile .NET 4.6 is in the making.
Core will however really form the "core", and other namespaces will be imported on-demand using NuGet packages.