I second the being able to describe a lesion. It's really impressive when you can use the right basic terms.
Even more important than anything else though, being friendly and charming. You want to relax (I know that can be hard when you are trying so hard to make a good impression) and show that you are a great person to work with. Derm isn't rocket science but you have a lot of clinic and a lot of face time with generally healthy patients...being likable is huge.
The first things that we remember most about rotating students is their enthusiasm and their friendliness.
Also, beware the residents that seem to have a chip on their shoulder....believe it or not there are a few that think they are all that because they got into derm....and just be professional, reliable, and friendly with them.