It's pretty much all canon, which is the biggest issue. Superman's story A can't contradict something in Batman's story B and Wonder Woman's story C, so all three writers have to be pretty much on the same page on details. And that's just three characters solo books, since all three characters are on a team together for story D that has to incorporate story A, B, and C. That's just with three characters and four books. Keep in mind there are many more characters and team up books and specials and alternate dimensions, so on and so forth. Now, there's no reason that story A has to be related to stories B, C, and D and can be off to the side, all that is important is that it doesn't interfere with what's going on in those stories. The big problem comes from "major events," where stories A through D are all connected, and how stories A through D leads into Event Story E.
The art is kind of a big deal, but the biggest issue planning on how these stories work with each other. That alone probably takes like a week. The best example I can give of this train wreck is to suggest reading all the current Batman books. Detective Comics, Batman, Batwing, Legend of the Dark Knight, Batman - The Dark Knight, Batman Inc., Nightwing, Red Hood and the Outlaws, Batwoman, Batgirl ...
George Carlin wrote:
I think it’s the duty of the comedian to find out where the line is drawn and cross it deliberately.