The general criteria that must be met for a homicide to be ruled justifiable are very similar in every state, including Florida. The best phrasing I have found so far is in Steve Johnson,
Concealed Carry Handgun Training, North Carolina Justice Academy, 1995, pp. 3-4, but these criteria would apply in every state, and definitely apply in Florida, Georgia, Kentucky, and other stand-your-ground states. (Note that like Florida, my state of North Carolina is also a Castle Doctrine state.)
(1) Justified Self-Defense
A citizen is legally justified in using deadly force against another if and only if:
(a) The citizen actually believes deadly force is necessary to prevent an imminent threat of death, great bodily harm, or sexual assault, AND
(b) The facts and circumstances prompting that belief would cause a person of ordinary firmness to believe deadly force WAS necessary to prevent an imminent threat of death, great bodily harm, or sexual assault, AND
(c) The citizen using deadly force was not an instigator or aggressor who voluntarily provoked, entered, or continued the conflict leading to deadly force, AND
(d) Force used was not excessive -- greater than reasonably needed to overcome the threat posed by a hostile aggressor."
(Emphasis added.)
Note that ALL FOUR conditions must generally be met in order for a shooting to be ruled justifiable (there is an exception for someone kicking your door in, but we'll get to that in a minute). A small handful of states used to add a FIFTH criterion to the list above, that of running away from the imminent lethal threat before turning to defend yourself (and hoping the attacker doesn't kill you while your back is turned). Florida was one of those states, and eliminated it with the new law; most states have never had such a provision to start with.
OK, the really important part. Where a lot of people get spun is the deliberate distortion of the phrase "reasonably believes" in self-defense statutes. Reasonable belief does *
NOT* mean merely "feeling threatened"; the phrase is a legal term, and its definition in the context of self-defense law is that in paragraph (b) above--i.e., that "the facts and circumstances prompting that belief would cause a person of ordinary firmness to believe deadly force WAS necessary to prevent an imminent threat of death, great bodily harm, or sexual assault." Merely "feeling threatened" isn't reasonable belief; the belief
has to be objectively rational, i.e. there is in fact a guy standing in front of you holding what appears to be a knife in a threatening posture.
Also, it is important to understand that a claim of self-defense is not an automatic exemption to the laws against homicide. Rather, it is what is known as an "affirmative defense"; unlike the regular innocent-until-proven-guilty standard applied to a criminal act, the onus in a self-defense shooting is on the shooter to demonstrate that the shooting WAS indeed justifiable self-defense. In other words, in a self-defense case, the standard is "guilty unless shown innocent," and if the shooting is questionable, it is much more likely to swing against the person claiming self-defense than it is to swing in their favor.
There are a few other conditions that may constitute justifiable self-defense; for example, there is a provision in U.S. legal tradition called the Castle Doctrine that says that if someone is making an illegal forced entry into your home (whether by door or window, whatever), you are authorized to use whatever force is necessary to stop them and it would ordinarily be ruled justified; the presumption is that if the guy is kicking your door down, he's not there to sell magazine subscriptions. A majority of states explicitly spell out the Castle Doctrine in their laws, I believe, but the principle is there in every state, even Massachusetts. Florida, and most other states, also allow the use of lethal force to stop a "forcible felony," i.e. rape, aggravated assault, armed robbery, etc. Texas used to also allow lethal force in defense of property in some circumstances, which probably dates back to frontier days when stealing your horse or your food would cause you to die of exposure or starvation); they may still, but it is my understanding that in general, shooting in defense of property in Texas can get you in big trouble.
OK, back to the new Florida law. The Florida law's primary effect was to eliminate the "duty to run away" provision, which was very subjective and had been abused by overzealous prosecutors; as I mentioned, most states don't have them, and a truly questionable scenario (you could have walked away in complete safety, but didn't) would fail points (a) and (c) anyway. The new also law reaffirmed the Castle Doctrine as it applies to your home, and also extended it to your car, so that if somebody actually tries to carjack you, you can use force to stop them as if they were breaking into your home (a response to South Florida carjackings which I think is reasonable). Finally, the new law states that if somebody attacks you and you use force in self-defense, and your use of force is ruled justifiable, the attacker cannot turn around and sue you for the injuries you caused him (yes, that happens). I'd refer you to Gutmacher, Florida Firearms: Law, Use, and Ownership as the definitive text on Florida self-defense law. The latest edition is current with the revised statutes.
Now, Georgia and other states didn't have duty to retreat provisions anyway, so the changes there seem to be mostly just clarification. I actually posted the Georgia statutes a while back, before the "stand your ground" law was passed and after it was passed, and the new law didn't legalize anything that wasn't already legal.
And in ANY state, if you shoot somebody just because you "feel threatened," you will go to prison for manslaughter to second-degree murder, depending on the mitigating circumstances, regardless of any Bradyite BS to the contrary.
The real innovation in the Florida law was in extending the Castle Doctrine (already law in Florida and most other states) to your car, so that if you were being carjacked, you could use countervailing force without having to first prove the carjacker meant harm, on the same grounds as if the carjacking were a home invasion, and that if you defend yourself and it is ruled justified, your attacker cannot turn around and sue you.