My first approved Photo on Dreanatime, is a famous monument of Italy. To accomplish this single photo it took me a whole evening, and several shots taken, just for a single photo.
This led me think how some time, some section and some subjects are hard to realize, specially if you have different kind of illumination and other situations of discomfort. Other problem is you don't have an idea what to realize.
From your point of view and experience, in this kind of image which one is the most hard problem solve to take a good photo.