Starting a community wiki to collect up objective best practices for plugin development. This question was inspired by @EAMann’s comments on wp-hackers.
The idea is to collaborate on what objective best practices might be so that we can potentially eventually use them in some community collaboration review process.
UPDATE: After seeing the first few responses it becomes clear that we need to have only one idea/suggestion/best-practice per answer and people should review the list to ensure there are no duplicates before posting.
