One of the most common reasons for slow shutdown is that the machine has a lot of software installed that run in the background, i.e. Steam, Skype, Messenger, Vuze, etc.
If you're not adverse to editing the registry (not for the faint of heart, and to be done with caution) and tend to close documents and applications before initiating shutdown, then you can
read this article for some good tips on speeding up the process.