Like a lot of people, I'm disappointed by the verdict in the Zimmerman case. However, based on my own experience serving on a jury, I'm not entirely surprised that it turned out the way it did.
First, on a jury, you have to find that the defendant is guilty beyond a reasonable doubt. I don't recall the exact instructions we were given on what constitutes reasonably doubt, but it's a tough standard to meet. In the case that I was a juror on, I know and I suspect that most of my fellow jurors knew, that the defendant was probably guilty. It was a domestic violence case; he had prior convictions. Statistically, he was guilty. But because of the specific details of the case, there was room for doubt.
Second, you can only find the defendant guilty or not guilty of the charge or charges brought against them. Second-degree murder has a very specific definition, and I'm not surprised that the jury didn't find that the prosecution had proven Zimmerman's guilt there. I'm more surprised that they didn't go for the lesser charge of manslaughter, but apparently they asked a lot of questions about that charge, so maybe that definition isn't as simple as it looks either.
I don't think any of the people on the jury I served on felt good about essentially saying to our defendant, "Okay, you're totally innocent, walk on out of here and hold your head up high." Sometimes I still wonder if we did the right thing. In a general sense, I think a justice system that reduces the risks of the innocent being wrongly condemned at the expense of occasionally letting the guilty go free is a good thing. Knowing that I was a juror who probably let a guilty man go free is a harder thing to sit with.