OK, Danito, I'll play along.

Hard to remember now, but I had not read the novel before I first saw the film, so I'm sure there were many things that were unclear to me.

Some were (I think):

1. How Vito knew that Barzini was his true enemy.
2. How Don T. knew that Michael's emenies knew he was in Sicily.
3. How Sollozzo knew that Luca was a spy.
4. Who Neri was and where he came from.
5. Why it was "smarter" for Tessio to turn traitor.

"A man in my position cannot afford to be made to look ridiculous!"