There are certain foods that just don't lend themselves well to slow cookery. Anything involving dairy items, such as cheese or milk, is a no-no. Dairy products will curdle if heated too long, and Bon Appetit recommends they be added at the very last moment. Another suggestion from the food magazine is to avoid slow-cooking pasta, since this is another food that wasn't designed to be cooked too far past al dente lest it turn to complete mush.
Also, avoid slow cooking seafood, since who wants a house infused with l'air de fish market? Gross. Not to mention, most types of seafood are better served by quick cooking and would tend to dry out in a slow cooker.
