The not guilty verdict for George Zimmerman in the Trayvon Martin case can most likely be attributed to a Florida law requiring the state to disprove beyond a reasonable doubt the defense of self-defense. It was impossible for the prosecution to do this given the absence of any eyewitnesses. A pretty good but not entirely correct analysis is by Andrew Cohen at theatlantic.com: Law and Justice and George Zimmerman.
Florida is not the only state whose law requires the state to disprove beyond a reasonable doubt the self-defense defense. Kentucky adopted a similar law in 2006.