If there is one thing at the PDC that has me excited about .NET, it is LINQ.
This is by far the most amazing feature to come out of .NET for middle-tier applications. In fact, it revolutionizes the integration of database queries by eliminating the need for plumbing between business logic and database code. The ability to query local data (arrays, maps, etc.) within the same query against a SQL server and perform joins and selects within that code is a powerful developer tool.
I’m very excited about this technology (in fact, so was most of the crowd that saw it), particularly with how it is totally integrated with C#.
Edit: Nice collection of LINQ resources here