A barriers to entry is anything that makes it difficult or impossible to take part in a project. David Neary from Neary Consulting has written a very good barriers to entry checklist. As always with categorization, I've ended up with a slightly different categories. For example, he lists lack of communication channels (e.g. mailing lists and IRC) as social barriers, whereas I consider those purely technical barriers.
For the purposes of this Wiki, barriers to entry have been divided in the following fashion: