From what little I've seen, the prosecution didn't meet the "Beyond a reasonable doubt" standard. Maybe she did it, maybe not, but you have to prove it.
From a common sense standard, rather than an American legal standard, she's obviously up to her eyeballs in her daughter's death and disappearance, but the vague facts of the case were in her favor, and prosecutorial incompetence broke her way.
My gut feeling is that the daughter died through neglect/abuse/gross-indifference (drugging her to allow the mother to go out on dates etc.) and then due to the mother's squirrely nature and as a known habitual liar, she needlessly over-complicated the cover story of a disappearance, to the point it actually came full-circle back into her favor by creating too much reasonable doubt.
Just through all the crazy that came out in the trial, I could see Casey Anthony actually trying to stage her daughter's non-intentionally (however negligently it may have been) dead body as a foul-play murder with the duct-tape by over-thinking all the lies. I'm sure many have seen the type, Narcissistic personality disorder, that continually manifests itself as constant calculation an scheming to the point of sociopathy as to what they want, what their status in life is vis-a-vi their family and peer groups etc.
She's one of those "goes free to protect the innocent" cases unfortunately.
