Remember the saying 'never work with children or animals'? That includes photography! But I'd be tempted to extend the definition when concerning photography to include pretty much all people in general. Most of the people you photograph will be kind, helpful and friendly, but you'll sometimes find a subject that wants their pictures taken a specific way, or at a specific time - but not the same as what you want!
Children and Brides are two great examples! I have some tips for Child Photography, but Laura Burlton has some great information on how to deal with uncooperative subjects on Photopreneur.