Does your archer always shoot an arrow at the enemy furthest away thinking that monster is the leader? Does your defender always attack the first monster he sees? Do your players constantly scan the battle map for hidden pit traps? Have you noticed that your players always use the same powers in the same order during every combat?
If you’ve answered yes to one of these questions or questions just like them then you have an incredible opportunity before you. Your players have presented you with the opportunity to design encounters based on their expectations and behaviour. As a DM you are about to have a great deal of fun designing and executing these encounters.