I can think of plenty of corners of .NET that I have visited over the past few years but I'd hesitate to call myself an expert in any of them... remoting, interop, COM interop, ADO, web services, ASP, forms, reflection, dynamic code generation, windows services, networking... It goes on and on. The joy of it is that it can get very complex as you try to bend the framework to work in different ways. Just looking at the .NET articles on this site demonstrates the range and scope of programming in .NET.