Actually, jurors are not supposed to be trained in the law; nor is it their function to understand the law. The juror's function is to listen to the testimony presented and determine the FACTS of the case. The law is determined and applied by the trial judge.
The beauty of the jury system is that it allows one to be tried by his social equals--his peers--who have no vested interest in the outcome of the case. The major problem with the system is that jurors bring their own opinions, prejudices, and values into the jury room, even though attempts may have been made to eliminate them. Even the most accomplished attorney cannot determine every juror's prejudices or point of view through voir dire examination. This fact was illustrated by a recent study performed by a leading research institute. Ten different juries were empaneled, each subjected to the same voir dire examination, and presented with the same hypothetical facts. Each jury determined a different award for the plaintiff. It should be noted that this was a civil matter, there is no evidence of a similar method of research applied to criminal cases.
Having said all that, I must remark that after twenty years as a practicing trial attorney, the jury system is not perfect, but is perhaps the best system that we imperfect human beings can devise. I want those who sit in judgment of me to be human, to know my passions, weaknesses, and to see me as a person, not an impersonal "defendant." Bottom line, there are issues with the jury system, but it is certainly preferable to any alternative available.
Jurors receive little to no training when they are selected for a trial, and are chosen mostly based on their beliefs and responses to screening questions vs. whether or not they are qualified. We can't really expect ordinary citizens to know the law, at least well enough to be just in their decisions.
As human beings, jurors tend to react emotionally to testimony and evidence as opposed to impartially as the law requires. They have families and jobs, and jury duty is usually considered an unfortunate inconvenience people try to get out of. It also leads at times, and I have witnessed this first hand, to a rush to judgement in the deliberation phase of the trial, as people are tired and they just want to go home.
The selection pool for jurors is registered voters, but some people seem to be selected much more often than others, so perhaps some more equitable system of selection could be devised.