A few bad things in code for which we have fairly consistent evidence:

• high nesting depth
• meaningless or single-letter variable names
• lots of code duplication
• very inconsistent formatting
• very complicated Boolean conditions with AND and OR
• functions with a lot of parameters

As a software engineering researcher, I strongly agree. SE research has studied code comprehension for more than 40 years, but for that amount of time, we know surprisingly little about what makes really high-quality code. We are decent in saying what makes very bad code, though, but beyond extreme cases, it’s hard to come to fairly general statements.