Well, if you bring a jury into a courtroom, they will bring their baggage with them. Predjudices, hate, fear, High or low iq, personal likes and dislikes etc.
Add a lawyer to the equation, and soon the trial starts to evolve around different things than it should be.
A good example (albeit an extreme one) of just how bad a jury system is would be the OJ trial. There focus shifted from the question of whether or not OJ was guilty, to the question "did the police plant evidence", "is that officer racist or not" etc.
The OJ trial is also an excellent example of what happens when you select stupid people for jury duty. The prosecution in the OJ case failed to explain the value of DNA evidence to the jurors, but they did try. The jururs were simply unable to comprehend that there is no better evidence than DNA evidence. Instead they got misled by the defense attorney
Lawyer
"So if you are saying that the odds of two people having the same DNA is more than 250 000 000 to 1, it could still happen"
Expert witness
"Uhh..theoretically yes but..."
Lawyer
"JUST answer the question thankyou"
Stupid juror #1 (thinking)
"Duuh, I suppose it could happen"