What makes good code good?

Assume that you've just become part of a team that manages a large code base for an interesting and fulfilling project. What most important aspects of the way the code is written will make the code good? What specific things are going to make your journey a complete breeze in terms of reading and understanding the code, adding new features, improving and patching it?

I'd highly appreciate it if you also indicated your experience and included an example from your past experience of encountering really good code (or bad), and what made it good (or why it wasn't).