Even though I've written before about the concept of Subjects in CRM, it can sometimes be difficult for me to get my head around the best way to create a Subject tree for a given implementation. Subject trees help to categorize cases, knowledgebase articles and sales literature in CRM, so you want them to be relevant. At the same time, I always resist the temptation to get too specific with them - they should be generalized categories, otherwise users likely won't drill into them enough and are liable to miscategorize things. Additionally, businesses change, and you don't want to have to keep editing and re-organizing your Subject tree. Still, they are useful, especially to service organizations that want to show related KB articles on cases.

Here's an article on Microsoft's site that has some sample Subject trees for a variety of organizations, from healthcare to banking, manufacturing to professional services: http://www.microsoft.com/australia/dynamics/crm/using/configure/subjecttrees_old.mspx. (Scroll to the bottom to find the samples.)