streams and network programming is most desirable, this also impacts other libraries.
Another big one is async programming. Some kind of unified, higher level framework to deal with this could be a huge help - it's a hard problem. It's interesting to see that in .NET a lot of new stuff concerning async programming is coming out, suggesting that the (many) existing patterns are considered inadequate. In C# 5, async operations will be a language level feature with the async/await keywords. There is also the reactive extensions library for async streams: http://msdn.microsoft.com/en-us/data/gg577609. Most interesting about this one is how easy it is to compose async operations.