ok here goes my best explaination. But before all of that i have to say you should have really posted after the comp, especially since this contest is ANONYMOUS (doh!). seconly these point are all just my humble opion.
Ok your first problem is your background and your hobits are at 2 different picture qualities, which is never a good place to start. it probably not your fault however as good quality mugshots are few and far between in my experience.
your second problem (but more of a minor one in this case) is difference in perspective. It would work better had you pulled up the hobbits so the center of the hobbits pic is in line with the center of the other pic and made more of a narative out of it. if it was my pic then i would crop off above the police thingy as its distracting from the main points of the picture and there isnt much happening there except grass.
third, is the randomness of the drain. its good that youve used the source picture obviously but it seems to be there with no purpose. if you need to, make it overly big so you lose realism but can see whats in the drain.
fourth, yes the lighting is different. Just brighten the mid tones up in levels and it may solve the problem.other than that im not sure, im not big on lighting.
In terms of enhancing it, you could add some atmosphere, some extra details or subtle changes which usually make all the difference.
the things you did right:
some kind of story going on in the picture which is always good. good intergration of the drain and you tried to intergrate the hobbits which many people don't.
as things go, you doing pretty well. you only pick up the finer points after you spend a long time on this site i think. definately a good start to your portfolio, everyone starts bad remember that. also, if you want more votes and comments then people seem to be more leanient in the non-prize contests, and the chances are they can tell you where youve gone right and wrong.
keep up the good work and dont expect everything to come to you instantly. we're here to help