As a medical transcription editor and transcriptionist I run across thousands of hyphens a day...many of them misplaced or inappropriate.
The biggest problem seems to be hyphenated or compound adjectives. The simplest description I could find for hyphenated or compound adjectives is:
"A compound adjective is formed when two or more adjectives work together to modify the same noun. These terms should be hyphenated to avoid confusion or ambiguity."
I found this handy little hyphen quiz. It's not hard, but interesting. See how you do (I got 12/12).