I've noticed two things that have ruined hackathons for me:
- People that say they have skill in XYZ, when in fact they have little to no experience in XYZ. It would be nice if I could have some reasonable confidence in a stranger's ability to be experienced in a skill they claim to have.
- "Entrepreneurs" interested in getting free dev labor for their unicorn app idea. I sign up for a hackathon to have some fun and write some code with hopefully cool people, usually for some cool or good cause. I don't come there to make a beta for some greedy MBA student.