Recently I had the experience of reviewing a project and assessing its scalability and maintainability. There were a few bad practices here and there, a few strange pieces of code with lack of meaningful comments. Nothing uncommon for a relatively big (legacy) codebase, right?
However, there is something that I keep finding. A pattern that repeated itself throughout this codebase and a number of other projects I've looked through. They could be all summarized by lack of abstraction. Ultimately, this was the cause for maintenance difficulty.