Copy (as in, words)
Lots of people take good copy for granted. Just like everyone thinks they can sing and do photography, everyone also thinks they can write. But there’s a difference between the way that you sing, and the way someone like Celine Dion sings. Same too, with copy.
Here’s a great slideshow where a professional copy writer critiques someone else’s website and gives great suggestions. I found it inspirational.
The author of that slideshow just launched CopyHackers (which I just read about on Hacker News). I think they write books about copy? (ironically, it’s not immediately obvious what they do from looking at their website.)