But Wikipedia covers why the plea system is horrifying is pretty great detail:
http://en.wikipedia.org/wiki/Plea_bargain
The American justice system from a foreigner's perspective just seems like a revolving door punishment system with almost no reform, and one where money plays a MASSIVE part. If you're rich you won't spend any time in jail, and you likely won't be convicted even if guilty. If you're poor you can be innocent and still have to plead guilty because you lack the $100K you'd need to mount a real defence (and you'd still lose).
I genuinely believe there are some good individuals in the US justice system, but most of the people at the top and the politicised prosecutors, politicians, and judges just ruin it. You could easily improve American justice tomorrow by scraping most pleas, re-introducing the right to a speedy trial (e.g. 12 months in jail to be found innocent is immoral/unjust), and increasing public defender funding significantly (at least 20x).