In reading all these posts, it does become abundantlyclear that Michael lacked his father's abilities to deal with potential enemies. As someone has pointed out, when Vito turns Sollozzo down, he does so with grace. He tells him he wishes him luck and success in his new venture so long as he does not interfere with Vito. I even think hepredicts Sollozzo will do well. Additionally, in GFII we learn that even though Vito never trusted Hyman Roth, they worked together and made a fortune. Look at how differently Michael treats people. When Geary threatens him Michael sneers and tells Geary not only will he not bribe him, but he wants Geary to pay the fee himself. Then Michael "muscles" Geary by having a prostitute killed and then blackmailing him. Faced with a similar problem, Vito had a horse killed and got what he wanted. As for Roth, there was no reason for Michael to virtually let Roth know what he was thinking when he pulled out of the Havana deal.
There was no need to confront him over Frank Pentangeli. If Roth posed a threat, why would Michael want to give him even the slightest hint that he was on to him?