so... after weeks of fighting in Najaf, after 569 Iraqis were killed and 785 injured, after attacking the most sacred Shia shrine, and after destroying parts of the biggest and most sacred Shia cemetery in the world... comes the "Grand" Sistani and solves the crisis in 10 minutes.

Isnt this a very interesting example of how to make heroes?

but why was he given the chance to be a hero, and what did he really do?

ok..
as for what he really did, he didnt do anything super
U.S army left Najaf, Mahdi Army still have the permission of carrying guns, and AsSadr is not an "outlaw" anymore, and he is invited to be an active "political" member in the Iraqi government.
So where is the great deal? if the U.S army left Najaf 3 weeks ago fighting would have stopped.

and..
for why was Sistani given the chance to be a hero
Everyone was exaggerating the importance of Sistani for weeks, Sistani Sistani Sistaaaaaaniiii, and then he comes back and stop the small Najaf war, so he becomes the Iraqi local god that can control anything, THEN he decides to ask Shia not to fight against the occupation forces.
Do what then? receive them with flowers?
At least, if he was a real national leader, he should have asked for something in return for this out-of-the-blue-peace.

The thing that the U.S administration miscalculated as usual is that they made a bigger hero of AsSadr as well, he is the one that protected the Shrine after all.

Miscalculations miscalculations...
The U.S administration miscalculates, and the Iraqi people pay the price.