I figured I would delve into this in an objective manner, and provide parallels in Missouri law.
There are three elements to prove 2nd Degree Murder in Florida: (1) the victim is dead; (2) the death was caused by the criminal act of the defendant; and (3) an unlawful killing occured in an immenently dangerous manner, demonstating a depraved mind without regard to human life.
(1) Trayvon Martin is dead.
(2) Trayvon Martin is dead because George Zimmerman shot him.
(3) The central factor in this trial comes down to the 3rd element of 2nd Degree Murder. There are three elements to demonstrate this: (a) an ordinary person would be reasonably certain to know the act would cause death or serious bodily injury; (b) the act was done with ill will, hatred, spite, or evil intent; and (c) the act itself indicates an indifference to human life.
(a) An ordinary person should be reasonably certain that firing a gun in the direction of another, at close range, could cause death or serious bodily injury.
(b) & (c) The trial will revolve around these two factors.
I do not have the facts of the case, and cannot make an objective assessment of whether (b) applies. Presumably, and in light of the prosecution's failure to include the lesser offenses in the Casey Anthony case, the prosecution would include the lesser offenses in this case. 'Culpable negligence,' as opposed to ill will et al., must be proved for a charge of manslaughter, but Florida's 'stand your ground' defense would probably negate this factor of the unlawful killing element.
As to (c), I am reasonably sure that a successful 'stand your ground' defense would also abrogate this factor and the element of unlawful killing, as the indifference to human life would be countered by the preservation of one's own life.
The 'stand your ground defense' requires that one "reasonably believes that such force is necessary to prevent imminent death or great bodily harm." Again, I do not have the relevant facts (and we can be assured the press does not have them either), but this defense would certainly be available to George Zimmerman. From what I understand, a judge could throw out the case immediately should that judge believe the defense is justified, but this will most likely be avoided as judges are elected in Florida (this is not an argument against abolishing the Missouri Plan) and a jury will likely determine whether the defense is justified.
In my personal opinion, if reasonable doubt exists as to whether George Zimmerman initiated contact with Trayvon Martin, I do not believe the prosecution will be able to prove 2nd Degree Murder. If the jury believes George Zimmerman caused a confrontation with Trayvon Martin, regardless of initiation, I believe a jury could find him guilty of manslaughter. That being said, the 'stand your ground defense' could nullify either charge.
Of course, none of this may preclude civil proceedings (think OJ Simpson), so one should not assume that a 'stand your ground' defense is an absolute bar to any punishment for George Zimmerman's action. Do not take my word for it, as I have not located the relevant statutes, but I believe George Zimmerman's 'stand your ground' defense would only be a partial bar to damages.
If this had occured in Missouri, 2nd Degree Murder requires that the defendant "knowingly causes the death of another person, or with the purpose of causing serious physical injury to another person, causes the death of another person." Missouri's standard appears a little easier to prove. And the 'stand your ground' defense only applies if the person asserting the defense was on property he owned or leased.
In Missouri, George Zimmerman would probably be found guilty under the known circumstances of the event. Now assume the facts show George Zimmerman acted lawfully in Florida, and ask yourself whether it would be just for a similar individual to be convicted of 2nd Degree Murder in Missouri, as a thought experiment.