I understand why you asked this question, but there really isn't an answer. Every program is different, and what makes one person happy may not make another. In general, here are some things that contribute to happiness:
1. Minimal scut work (for example, nurses do prior auths and most of your call backs)
2. Protected time for didactics
3. Attendings see patients too (some of you may disagree, but the dynamic between residents and attendings is different when attendings also see patients)
4. Happy attendings (some famous dermatologists have famously terrible personalities. You may want to train with the world famous Dr. X, but it turns out Dr. X may be a total jerk. Programs with happy attendings also have happy residents)
5. The culture of the department (some places do a better job of making the residency and work environment happy and pleasant--this may sound odd, but it is true)
6. A large department (in residency, people get sick, people get pregnant, people have life events--if you are in a larger program, it is easier to spread out being down a resident)
Are all "top" programs malignant? No. Are all "lower-tier" programs cushy? No.