An issue that we have today regarding COM interop is the deployment of Primary Interop Assemblies (PIO). An application that uses Excel 2007, the primary interop assembly required by the application requires 1.2 MB. In reality you need to install the Office 2007 PIA Redist that requires 6.3 MB. And this is just for Office 2007. The next version requires another PIA redist install.
This is one of the issues solved with the new type embedding feature.
With type embedding PIAs are no longer required. Type embedding is possible by the new CLR and a new compiler switch.
The second feature for the CLR type system is type equivalence. Type equivalence is useful both with COM objects and .NET managed code. Interfaces marked with an GUID attribute are treated equivalent if the same GUID is used. Multi-targeting through interfaces. Does this influence System.AddIn? I've to try this out!