For a general explanation of exactly what plea bargaining involves, see this article.
Whilst we do have a system of negotiation here in South Australia, this is not done with the direct assistance of the Judges.
The other main difference is that the DPP and Defence Lawyers cannot "agree" a specific sentence that is then rubber-stamped by a Judge. That of course runs a little contrary to our strongly held principals of judicial independence, and that is one of the arguments against such a system.
The one thing that an accused person often craves is certainty. The uncertainty of the sentence that might ultimately be imposed under the current system is a dis-incentive to some accused people pleading guilty to certain offences.
My clients will frequently say things like, "If you can guarantee I'll get 4 years, then I'll plead guilty," but in our current system such a guarantee is not possible.
Even the usually conservative English legal profession has been examining the possible benefits. See this article.
Plea bargaining: not perfect, but it works - Times Online
British judges, lawyers and prosecutors have spent two years hammering out the finer details of how to introduce a similar system here.It would not be the solution to all of our problems, but it just might help.