I think the lack of "dangerous" characters is a natural progression of the Trilogy which I feel, in a general way, parallels the immigrant experience in America.

Step 1: Street-level, culturally isolated.
Step 2: Moving up in the world, attempting to overcome resistance to integrate into mainstream, "legitimate" American society
Step 3: As powerful as any "non-ethnic" American, and not held back by prejudice on a daily basis, but still struggling to move past the choices they had to make to rise past steps one and two.

It is clear and understandable why there was no one as "dangerous" as Sollozzo in the latter 2 films. But I can't say I agree that the emotional impact of the villians' actions is any less in GFII and GF III. Witness the killing of Fredo and the silent scream.