The deadline in general. Imagine having a great idea for an entry, but you have to cut it short because of bad-time-management/real-life-issues/general-lack-of-time. You want to make it the best and yet time limitations bring it down. In my case, it's totally my fault that I don't practice decent time-management....
I voted for coloring, because I kind of failed with my timing this time. I guess everything else can be also included, with exception for backgrounds - come on, sometimes it's as enjoyable to draw bgs as the characters. And oddly, it's the hardness of making a good comic in general that makes it such enjoyable task. I mean, it's a shitload of work, and yet we're all doing it instead of sleeping at nights and such... we're all loonies here
ALL OF THE ABOVE!!! nah...with my limited experience, the hardest thing I found was making it simultaneously an exciting battle but also a great story...I came a cropper on that when I was battling last week. A good storyline and dialogue can very easily beat good artwork I think, unless your judges are 'all about the art' you know?
Well, since I've never entered one, I'm going to guess what would be the hardest for me. Backgrounds. I can't draw them to save my life Well, I can't draw anything to save my life, but backgrounds are the absolute worst.