Be aware of Applications, It may be there has some old Windows applications. While Windows XP can manage to run many of these applications, but in Windows 7, may have problems natively running those applications.
Incompatible Hardware, Windows XP supports some old HW, but be careful in Windows 7, when you are trying to run them on 7. You have to concern about that, and have compatible drivers also. Deploy Windows 7 with Windows Deployment Services or WindowsPE with ImageX. Migrate the users settings using USMt and update with logon scripts or GP.
Application compatibility Toolkit or ACT helps to install, configure and care about security related matters. You can find it out here.
You may also go for XP Mode.