Well,let me ask you this.
What did you expect out of Leonardo this year?
They sold their best player in Kaka and basically did nothing to re-inforce the squad, and he has managed to get a decent season out of Ronaldinho.
The average age of that squad is ridiculously high, and he has managed to squeeze everything out of them that he could.
What were the pre-season expectations of this squad. At best, 3rd?? Some prognasticators had them at mid-table, considering what he was left with.
Add in the fact that he works in a pressure cooker in Milan, and it is his first year as a coach.
I think he has done a lot better than what most people expected.
He's young, he's forward thinking, likes to play exciting football, and most importantly to a lot of people here, he is not Italian.
What I expected of Milan & Leonardo was finishing 3rd (behind Inter & us) => looks very likely at the moment.
As for the CL, I expected them to pass the 1st round fairly comfortably & possibly go as far as the quarter finals or even semi's, depending on the draw(s) obviously. But since they were partnered with Man U pretty early, it was only logical for them to go out.
Milan (and Leonardo) has achieved exactly what I was expecting. So for me that's an ok season, nothing more, nothing less.
Though I will admit that Milan this season has usually looked better than last season. Which in the end means very little, if anything.
And to answer your initial question: I do not think he would be the right guy for Juve right now.
His Milan ties would mean that a couple of (inevitable) bad results would lead to an immense pressure (from media & fans alike), prohibiting him to do his actual job. Much more so than it would be the case for an outsider.
Furthermore, he also doesn't strike me as a born authority figure & he has little experience, which makes it even harder to command authority.