Re: What's the worst that can happen?
1. A failure to adapt Batman into the MOS world. It shouldn't be that hard to succeed at making Batman awesome in this film, he doesn't even need much screen time to be developed properly. Snyder will shoot him with amazing visuals. However, if all this goes awry..it will derail the movie. If Batman isn't cool, believable within the world and feels shoehorned in...you'll lose the viewer on the entire concept of the film.
2. The villain will be underdeveloped. I fully expect this to happen. That's why they need a strong A-list name for Luthor IMO (assuming he's the bad guy) He won't have much time to develop so you need someone with gravitas. Someone who commands a presence without even uttering a single line. A Luthor with a big name attached (Matt Damon would be my out of left field pick) would be able to go up against Bats and Supes in a believable way without a lot of screen time to do so. If the villain is weak, the conflict between Bruce and Clark that builds into their natural partnership to go against said villain won't work very well.
3. TOO much time spent on Batman. I believe Batman should just cut in and out of the story. He should be the scene stealer, not the character that drives the film. He should be dark, mysterious, funny and awesome but he shouldn't be the focus. He doesn't need a character arc besides his relationship with Clark being built. This point leads me to my final one.
4. Superman gets the shaft. Everyone has already commented on this. I'm a Batman fan above all, but this should be Superman's film with Batman featured.