Having Juries decide criminal cases = 
Getting a group of 12 people to pass judgement on laws they know nothing about,without any bias etc is never going to work. Not now not ever.
From personal experience as a Juror,stories from others who have been jurors, and seeing things like this happen.
Actual calls from aJuror rooms:
"well the police probably planted that evidence there"
"I dont trust her (the victim) she seems like a slut"
"I'm going to vote not guilty just to piss you off"
"It was probably an inside job from the dodgy bloody [insert ethnicity]"
"he/she may not have done this crime,but he/she has certainly done something else wrong"
"I'm voting not guilty,I couldn't vote guilty because he/she is wearing a mask in the footage,I'd need to see his/her face on camera before changing my mind" (ruling out 99% of guilty verdicts in all robberies)
There have been many more....