I think one of the most important skills of a programmer is the ability to look at the source code, and be able to tell what it does. Not by comparing it to similar code they have seen before and guessing, but by going through the instructions step by step in their heads. There are programming languages and tools that are designed to make that easier, and I always hugely preferred those. Not being able to do that, because it depends on context or is not well defined is a major problem for me.