Hyphens, en dashes, and em dashes -- a lot of people don't realize that these punctuation marks are not, in fact, equivalent. The difference between hyphens and dashes is actually really important: a hyphen makes two words into one word; dashes can add a greater division between words and even entire phrases.
Back in high school English, I took it upon myself to formally learn the differences between these three similar punctuation marks and tried to use them to their fullest in my writing, because I noticed this kind of detail in the books and essays we were reading at the time and I wanted to master that particular detail for myself. I wouldn't say it's exactly a pet-peeve of mine now, but when I see them used incorrectly, I definitely notice.
The difference between en dashes and em dashes is less important in practice (you can use an en dash -- or even two hyphens -- with spaces around them to fill in for an em dash when you don't have an editor capable of letting you input the correct characters, and readers will more or less know what you mean, but in editing for a printed work, it's best to replace the hyphens with en dashes or em dashes where appropriate).
Here's a great description of the differences:
http://www.chicagomanualofstyle.org/qanda/data/faq/topics/HyphensEnDashesEmDashes/faq0002.html
