The real answers about complexity come from thinking about why we even care. It's because our feeble minds have to build internal models of the code so we can work with it. The cognitive aspects of building those models is why complexity matters.

What things make it more difficult to build those models? A partial list, mostly as others have mentioned:

- tool and library dependencies - nested conditions - loops and especially nested loops - asynchronous processing, callbacks, etc - non-descriptively named variables and functions - using non-standard code patterns for standard functionality - delocalized code, as in, you have to navigate somewhere else to see it (throws off your working memory)

By one study, developers using Eclipse for Java spent 27% of their time just doing code navigation.

The starting point for code complexity is about how our minds work.

As many people have said, "it's easier to write a program than read it."