if you wish to write a letter, be polite yet direct about your feelings for Greyhawk. Point out what you do and do not like about Wotc's current products and emphasize your desire for new products based on D&D's classic setting. If you like, you might mention a compilation of the Living Greyhawk Journal (one idea that has been floated recently) as there are a number of people talking about it as a realistic goal for Greyhawk. Lastly, you could mention that the RPGA's underfunding for Greyhawk is unacceptable and that Wotc should attempt to remedy that financial shortcoming.