This is coming from someone who hasn't graduated med school yet, but when I think of derm, I think of acne, rash, acne, acne, eczema, psoriasis, acne, mole check, eczema, acne, melanoma, acne... I know there are many minutiae they have to learn and that dermatologic skin diseases are very important for quality of life, but I just could not get myself interested in the bread and butter complaints that dermatologists address. Not to mention that manual instantaneous fermentation of liquids and low-speed unpowered water-skiing during storms are pretty much pre-requisites to matching in derm these days.
I tried radiology since my parents kept hounding me to do so as they heard it was the big bucks, but while I found the technology pretty cool, the monotony on a minute to minute basis, the lack of patient contact, and the fact that you no longer handle medical decision making axed that specialty for me. I found radiation oncology to be especially cool given the technology and because dealing with social issues such as death and dying have always been an interest of mine but ultimately decided that I did not want to work with dying patients nor give them complications such as pneumonitis, erectile dysfunction, and colitis. Urology is similar to ophtho in several ways and I enjoyed that rotation, but I also felt uneasy about the complications and questionable utility of routine surgical intervention in prostate cancer. ENT has a few similarities to ophtho but with much more involved, longer and inpatient surgical cases. Internal medicine is pretty cool as well because there is a lot of variety and social issues to manage, and there is a wide range of subspecialties or general practice patterns to pursue.
I remember reading somewhere that there are over 110 different ways to subspecialize in medical care -- probably best to make your decision about medical school first and then go from there, though I agree that it is a good idea to think ahead about what your ultimate goal and endpoint might be along the way.