You make an important point. We need to remember that in the book, Mike was viewed as "yellow" or weak by the other Families. Even in the movie, the line "Sollozzo knows he's a civilian" implies that Mike is not in the business, nor is he considered strong enough to be in the business. Why else would Sonny, Tessio & Clemenza all laugh at Michael's suggestion to kill McCluskey & Sollozzo?

The view of Michael (before the Sollozzo murder) is that of a non-factor within the Family. This view is seen by the other Families, as well as Clemenza, Tessio, et. al. With Sonny dead and Vito ailing, the Corleone's were deemed vulnerable by their enemies. No one initially expected much out of Michael when he was given the power. Remember Tessio argued with Michael's decision not to recruit more men: "Mike, you're wrong."

Therefore, the execution of the plan to kill the heads of the Five Families, Moe Green & Carlo was most likely a shock to the mob underworld. Obviously, they all knew who had ordered the killings since Michael was still alive. I think that they greatly underestimated his cunning & planning and realized that he was as intelligent (or maybe moreso) than Vito and that Michael was a forced to be reckoned with.