Silverlight
The main advantages of Silverlight (and by extension, WPF) are described in one of my articles Silverlight 2.0 In Examples: Part 0. Introduction. As detailed in that article, Silverlight's primary benefit lies in enabling developers to cleanly separate between the visual representation and the underlying business logic. This is achieved by techniques called binding and data templates. This is why, I think Silverlight is more powerful than its main competitor Adobe Flash. While Flash is great for creating fancy rich content, it is not widely used for building business logic applications. Silverlight, however, will be widely used for both.
WPF
Unlike Silverlight (which is used to develop browser based applications), WPF (Windows Presentation Foundation) is the new framework for creating stand alone applications on Windows platforms. It is almost a superset of Silverlight, so, almost everything that applies to Silverlight also applies to WPF. WPF has many features that Silverlight does not have (yet), but the WPF applications will only run on Windows with the required version of .NET framework installed, while Silverlight can be run in multiple browsers on both Windows and Macintosh platforms.
LINQ
LINQ is another cool new technology allowing the developers to insert SQL like statements into the C# or VB code. It makes it much easier to access and modify data within collections. From my point of view, LINQ is a must know for every C# software developer who wants to be efficient.
WCF
WCF (Windows Communications Foundation) is a glue that ties different parts of an enterprise system together. WCF technology allows changing the underlying protocol e.g. from an all penetrating XML based SOAP to faster binary protocols without any changes to the code. With multi-billion Microsoft muscle behind it this technology is likely to dominate the Windows world.