Everything was weaker
- Compare the opening parties in GF I, II and III. The third one could be anybodys. Can you imagine anyone in GFI bumping into the singings doing their "Zamsa-sam"? and anybody would be annoyed?
- It's not the plot IMO, the "lot" in GF II is also full of gaps. It's that you have hardly any reaaly good and memorable scene.
- The actors. Well, most of the OK., but in GFI, they are all great.
Just compare:
GFIII - GFI
Zaza - Sollozzo
Vincent - Sonny
Michael - Michael
Anthony - Fredo
Mary - Tom