The tech stack
Crappy/lacking/out of date documentation
Can't get the project up and running locally
or it's way too complicated a setup
Unfriendly/condescending contributors on the project
Not wanting to deal with someone's mess (poorly architected solution)
Poorly maintained/managed project - coming to a project and having no idea where it's at, or if it's even actively being maintained.
Complete overwhelm in not knowing where to dive in!
Lack of development workflow, coding standards, and/or too many "cowboys" on the project (ie: no team cohesion).
I've been looking for a while now for an open source project I feel comfortable and motivated to contribute to. Hoping that knowing what prevents me from working on others projects, that the upcoming open-source project that I want to start will be more welcoming to new open-source contributors.