What's new in the BCL in .NET 4 Beta 1 (from BCL Team blog)
Interesting stuff : http://blogs.msdn.com/bclteam/archive/2009/05/22/what-s-new-in-the-bcl-in-net-4-beta-1-justin-van-patten.aspx
My favorite features are parallel extensions, code contracts (I was familiarized with them thanks to Peli ), memory mapped files and compression improvements. Hell, any improvement to the current state is great! :-D And wait till you see the list of ASP.NET improvements. Scott Galloway has already blogged about a few of them...