First of all, let me say that I like Ruger Firearms. I just never took to the Mini. I don't know too many people that have. You can't do much to them to make them any better either.
What is it that gives the AR much better accuracy?
Skans pretty much covered all that.
The AR is more accurate out of the box, and there are a few things that you can do to it to make it even better. You can Free Float the barrel for a couple of hundred dollars. You can modify the trigger for free, or you can replace it for $100 to $400.
What is it that the Ruger is lacking, and what can be done about it?
Again, Skans pretty much covered it.
The barrel, the stock, the trigger, and that damn barrel band thing. The barrel is too thin, the stock actually puts tension of the thin barrel, and the barrel band securing the stock to the barrel creates more tension on the barrel. I think Ruger finally came out with a new adjustable trigger, but there are too many other things going on with the rifle that negate it's advantage.
DBAR