I know this isn't advice that everyone will approve of, but I've found putting a bit of warmth into the cover letter can be helpful. You don't want to sound completely cold and robotic. Of course, you don't want to sound too lighthearted about it either. It's best to try and find a balance between the two.
When it comes down to it, the people reading the letter are human and they have a lot of letters to read. Giving them something genuine to remember you by can't be a bad thing.