Cleanliness
Most of the Hogwarts' House Elves had gathered around a cauldron in the Kitchens, each of them holding a bowl of soapy water. They took a slip of paper from the cauldron, read it, and disappeared with a small pop. Dobby and Winky were patiently waiting for their turns.
Dobby took a slip, read, and groaned.
„Slytherin Sixth Year boys! They just have Quidditch practice! They muddy! They smelly!"
„Dobby do his work!" Winky admonished. „Dobby not groan. Dobby wash children." She quickly grabbed a bottle of make-up remover from a nearby table and disappeared to Ravenclaw Seventh Year girls.
NOTE: Not exactly inspired by DH, but just by the fact that the characters seem to wash themselves neither before bed nor in the morning.