Team Foundation Server work items have an assigned to field that defaults to the Valid Users Group. The problem is this group includes service accounts also.
You can easily change the list to contain just the groups you want.
Open up the work item with the Process Editor you get when you install the TFS 2010 Power Tools.
Open the field definition for the Assigned To field in the work item and switch to the Rules Tab.
Remove any rules that are there now. and add Allowed Values and Default.
Edit Allowed Value and set it to the following to get all the project administrators and Contributors along with an unassigned value. Make sure you exclude Groups.
Under Default set the value to be Unassigned.
Now you are all set all that will show up in the AssignedTo drop down is Unassigned along with Project Administrators and Contributors.