I think a lot of the questions about the Florida SYG law revolve around the provision that:
(2) A law enforcement agency may use standard procedures for investigating the use of force as described in subsection (1), but the agency may not arrest the person for using force unless it determines that there is probable cause that the force that was used was unlawful.
IMHO, this is one of the most important provisions of the law. It protects someone with a reasonable claim of self-defense from the expenses of defending his or her actions in court. While this is in some respects a laudable attempt to protect the innocent, it seems to substitute the findings of investigating police and prosecutors for those of a jury.
In the Zimmerman-Martin incident, it appears there are substantial questions regarding the facts of the case which should be presented to a jury.
Violence is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and valorous feeling which believes that nothing is worth violence is much worse. Those who have nothing for which they are willing to fight; nothing they care about more than their own personal safety; are miserable creatures who have no chance of being free, unless made and kept so by the exertions of those better than themselves. Gary L. Griffiths, Chief Instructor, Advanced Force Tactics, Inc. (Paraphrasing John Stuart Mill)