...for you to feel it was a good movie?
I think Cuaron's probably felt the pressure of this movie...a lot of people name book #3 (even after book 5 came out) as their favorite book, and they'll want it done right. There are so many good parts to book 3 that it's hard to know what to leave out! I know, for me, I would like to see the following things in movie #3 and done well (from book 3):
(1) the confrontation in the Shrieking Shack
(2) the dementors
(3) Harry's flashbacks and patronus
(4) lots of Lupin/Harry interaction
(5) flashbacks of the Marauders as well as seeing the Marauders' Map
(6) Hermione slapping Draco
(7) Hermione being snarky in Diviniation (I read somewhere that Emma Thompson is playing Trelawney...THAT should be interesting...she (Emma) seems too likable!)
[8] The time-travel at the end and Hermione trying to get to all those different classes
There are probably other things, but those are the first that came to mind. There is so much good stuff in book 3! What are you looking forward to seeing in the movie?