Source: Peter Brady's Blog
Text rendering has always been Silverlight's dirty little secret. For every major release I hope that maybe this time fonts won't look terrible. Unfortunately as of 4.0, there are still a lot of improvements that must be made before Silverlight text will match the quality and readability of Flash. Robby Ingebretsen gives some helpful tips that can improve smoothness in Silverlight 4. But one of Silverlight's most basic controls continues to look terrible.