Code Litmus Tests

Here are some questions you can ask yourself when evaluating the code you write:

API Modularity: Can you describe well your API, if written in a purely human language with no code extracts?
Compactness: Does an experienced user need a manual?
Orthogonality: Does changing a part of code affect other system properties?
Single Point of Truth: Do data structures have states with 1-1 correspondence with the states of the real-world system?
Code Modularity: Are there any global variables? Is the size of modules too large? Are there any large functions? What is the call maximum stack depth (excluding recursion)? Are there many internal APIs? What is the number of entry points to the module?
Transparency: Are there any special cases? Are there any magic numbers? Are each function calls orthogonal? Are there many mode flags? Is the high-level state of the system easily inspectable? Can you see what the system is doing through any debug output?

Others:

Are you using a binary format? Have you considered the pros and cons of that, against a simple textual format?