The following two are my favorite features comming with .net 4.0 and Visual Studio 2010:
- parallel extensions – a great and easy way to utilize those idle cores in your multi core machines. It might be as easy as adding an AsParallel() extension method to a LINQ query. I even did a bunch of presentations on the topic, the last one at NT Konferenca 2009.
- code contracts – improving the code in both design time (static analysis!) and runtime (note: albeit this feature is a part of .net 4.0 beta you have to download the package to install design time support for Visual Studio 2010 beta. This step is supposed to be redundant for RTM or sooner).
Perhaps there are other better features however these two are the ones I am most interested *currently*. So, what are yours?