In our training as Dermatologists, we cover many exotic diagnoses. We have to know as much about skin conditions as possible, but in real life, we see many variations of common conditions. Acne, Rosacea, Eczema, Psoriasis, and Skin Cancers head the list for me. This would also include the many manifestations of long-term sun exposure such as lentigos (brown spots), and Actinic Keratoses (precancerous areas in Sun exposed areas of skin).
Long-term sun exposure results in many skin changes, some of which are just related to appearance, but some changes from Sun can be deadly. I spend at least 20% of my day treating the effects of long-term sun damage.