I'm very sensitive to certain quirks in code formatting, mostly inconsistency. Inconsistency makes it more difficult for me to read code, so it slows me down when I'm programming. Not much mind you, but enough that I notice.

The one inconsistency that really gets me is mixed tabs and spaces for indentation. Having tabs on one line, spaces on others makes me shudder every time... A mix of both on the same line just makes me want to smash something.