There's also waterfall vs. incremental approaches. Most (successful) open source projects can be added to easily. That is, they are modular, with a basic core

There's also waterfall vs. incremental approaches. Most (successful) open source projects can be added to easily. That is, they are modular, with a basic core architecture, so a contributor can discover then understand then modify the part they need to - and those projects that aren't modular will be replaced by those that are. There is an assumption here that the concept of the project is understood well enough for a workable architecture to be known - without this, it's harder to add contributors, because they must first grasp the project as a whole.