Re: The origin of the jury
"originally, they were the people that knew the plaintiffs / defendants and in theory would know the background to the case"
That sounds very much like "knowing a lot of gossip that wouldn't be admissable". Surely it was the *witnesses* that the juries were supposed to know (and judge the credibility thereof).
But, yes, all that is long past. (Would knowing one of the witnesses be enough to disqualify you these days?) Juries, however, remain and amongst the reasons cited are their occasional tendency to deliver an unexpectedly verdict. On further questioning, the supporter of the jury system usually seems to be thinking of cases where the jury put equity before the law rather than following it.
On the other hand, it may be a little like democracy -- the worst system except for all the others. For "safety-critical" systems, the most important design criterion is not "How well can this system perform on a good day?" but "How badly wrong can this system go on a bad day?". Amongst systems of government, democracy wins convincingly on the second point.