Where to Start
If you need to learn more about Silverlight 2, be sure to check out the following sites:

http://devx.com/ria
http://silverlight.net

There are plenty of links and resources to be found at these sites to help you learn more about Silverlight 2.

Types of Games

If you're not sure of the type of game you want to build, check out these links for some ideas:
http://en.wikipedia.org/wiki/Video_game_genres
http://www.ringsurf.com/online/1670-genres.html

It's best to consider a game that works well with a hosted service and one that isn't dependent upon the CPU or speed of the network for performance. This will give your game the most appeal to the widest possible audience.

Silverlight 2 Features

It is impossible for us to list all the features in Silverlight 2 or which ones should be used for your game. Some (but not all) of the features of Silverlight 2 to consider using are:

Layout manager support
Two-way data-binding
Control template and skinning
Core form controls
Built-in layout management
Common functionality controls
Data manipulation controls
Cross domain network access
Collections
I/O
Generics
Threading
Globalization
XML
Local storage