It took me a while to figure out that what was bugging me about a particular design’s stylesheet was the mix and match headings in two different fonts.
I don’t mind if people style their fonts for their content paragraphs differently from the font for their sidebar lists (I do that here on Hoyden myself), but when some headings are serif and other headings are sans-serif in a moderately haphazard fashion? That’s. Just. Wrong.