But how do you decide on a user? Do you have only one user to assign tasks to?

Great!

I see you are using this query:

SELECT %Actions, %Message, %Priority, %Subject, TaskStatus_TimeCreated, ID 
FROM EnsLib_Workflow.TaskResponse 
WHERE TaskStatus_AssignedTo = ? 
  AND TaskStatus_IsComplete = 0

So it returns only uncompleted tasks assigned to a current user (by the way UserName is a valid sql variable so you don't need to pass it as an argument from ObjectScript). It does not return unassigned tasks - do you autoassign tasks in production? If so, how?