Ooh, I hate the use of spaces instead of tabs, too. And particularly hate it when people don't close their img and br tags with a </> !!

On a personal note, I get so frustrated when people don't add commented-out detail to their CSS! Especially when more than one person writes the code!

God... on that note, I'm endlessly frustrated with people that add a new line contradicting a class or id attribute stated above... like let's say line 20 states .class {margin: 4em} and then someone adds .class {margin: 0} on line 300! Oh, that makes me so frustrated...

Here's my quirk, though... Always writing in the default attributes (like padding, margin, and z-index). I'm sure that extra code bothers people I work with, but in my mind, it's important to be thorough....