Now, here's where its cloudy for me. Was the killing of Tattaglia, Stracci (sp?), and Cuneo just to prevent them from coming after the Corleones for killing Barzini, or was it something else???

If I'm not mistaken, only Barzini, Tattaglia and Moe Green were killed in the novel. Tattaglia was killed as further payback for Sonny's death (despite the fact that Tattaglia's son was already killed). Moe Green was killed because of the way he handled Fredo and Barzini was killed to stop the threat against the Corleones.

I could only suggest the others were killed to juice up the movie.

The sequence in the novel always bothered me a little because it seems that by getting rid of Moe before they made the move against Barzini and Tattaglia , the Corleones might be showing their hand a little bit.

I mean if I were Barzini and I'm pushing around the Corleones, taking over their gambling and loan sharking rackets, chasing them off the docks and maybe starting to cut them out of their narcotics payoffs and they do nothing, then I will think they are soft and weak. But if I hear that some new Corleone hitman who no one's heard of before takes out Moe Green, then maybe I should reassess, no?

Moe was small potatoes. Had Barzini's attempt to kill Michael and become the big cheese been successful, he would have never kept his end of the deal with Moe Green.

So Barzini may have not viewed Michael's move against him as any kind of threat to him. In fact Barzini probably figured, "good, Michael Corleone has done my dirtywork for me." Barzini's thinking was probably that since Michael moved Moe out, now all he had to do was take Michael out and he'd automatically be the Capo Di Tutti Capi and the casino that Michael had taken from Moe would become his (Barzini's) without his having to take out Moe Green himself. His thinking probably was that "The Corleone's have given me one less thing to worry about by killing Moe Green."

Remember that at that point, in Barzini's mind, the Corleone's were powerless. At that point, Barzini had already began seeing himself sitting on the throne.

Moe was small potatoes. Had Barzini's attempt to kill Michael and become the big cheese been successful, he would have never kept his end of the deal with Moe Green.

It's not even clear that Moe actually had a deal with Barzini. "I talk to Barzini. I can make a deal with him and still hold onto my hotel..." Moe might have been bluffing Michael at that point. Or he might have inferred that he could make a deal with Barzini, without actually having broached the specifics with him.

Tattaglia was killed as further payback for Sonny's death (despite the fact that Tattaglia's son was already killed).

Wasn't Barzini responsible for Sonny's death? It's what Carlo stated before he was whacked, that he was confronted by Barzini. Plus the Don said it to, "... it was Barzini all along" after the commission meeting.

Something I don't understand is why would Michael make such a bold move in eliminating the heads of the five families, to move to Nevada? What's the point in that? He had the city, and the cosa nostra, in the palm of his hands. I could understand expanding your influence to Nevada, but moving it entirely?

[Wasn't Barzini responsible for Sonny's death? It's what Carlo stated before he was whacked, that he was confronted by Barzini. Plus the Don said it to, "... it was Barzini all along" after the commission meeting.

Something I don't understand is why would Michael make such a bold move in eliminating the heads of the five families, to move to Nevada? What's the point in that? He had the city, and the cosa nostra, in the palm of his hands. I could understand expanding your influence to Nevada, but moving it entirely?

Besides the movie scene,that is totally served by a sequel of murders while Michael states that he declines Satan and all his acts (tremendous filming),we have to consider Michael's character,which is best described in #2:
Michael was the kind of guy who never takes any risks.In order to be the leader,he doesn't hesitate to "sacrifice" or better not to forgive,even his on brother.When Tom asks him "You won Michael.Do you have to kill them all?",he replies "Not all.Just my enemies".His power is his reason.If any of the heads of the 5 Families survived,they would run after him.They might,but he doesn't want to take the chances.
Except only one time,for Michael it was not personal,not revenge,ont infamnia,not honour,it was just buisiness.

Quiz/pole: which was this very unique time?

Besides the movie scene,that is totally served by a sequel of murders while Michael states that he declines Satan and all his acts (tremendous filming),we have to consider Michael's character,which is best described in #2:
Michael was the kind of guy who never takes any risks.In order to be the leader,he doesn't hesitate to "sacrifice" or better not to forgive,even his on brother.When Tom asks him "You won Michael.Do you have to kill them all?",he replies "Not all.Just my enemies".His power is his reason.If any of the heads of the 5 Families survived,they would run after him.They might,but he doesn't want to take the chances.
Except only one time,for Michael it was not personal,not revenge,ont infamnia,not honour,it was just buisiness.

Quiz/pole: which was this very unique time?

Why, with possible exception of Barzini, were the Dons without protection at the time they were murdered?

They were slippin'.
They all underestimated Michael.