Any lawyers here think jury instructions may have affected the verdict?
Not really. No DNA on the duct tape, no DNA from blood in the trunk, only witness testimony regarding a smell in the trunk vs. an expert on bodily decomposition saying that a body could not have decomposed in the trunk, no cause of death...murder is just impossible to get on that evidence.
Manslaughter? You still have to prove she caused the death, and when you can't even prove that she was around the body before or after it started decomposing, then there's definitely reasonable doubt for manslaughter.
Negligence? Well, sure, if we know what caused the death of the child and can show that someone's negligence led to her death, but we don't even have a cause of death, much less any evidence showing that anyone's negligence caused the death.
Abuse? Again, no evidence. Caylee was alive one day, and was found as a decomposing body the next. No DNA links Casey to her body. Forensic evidence and testimony gives different accounts as to whether Caylee's body was ever in Casey's trunk, and even if it were, there is no evidence showing who put her there.
It simply boils down to there not being enough evidence to rid the average person of reasonable doubt.