The biggest one is whether or not Alfred loved Susannah. Or did he just see her as a trophy wife?
Additionally, there's Alfred's claims of having "followed all the rules" and status as the good son, despite his association with the O'Bannions. The first time he met them, it was clear they had some sort of "arrangement" with the Sheriff and they weren't exactly secretive about their involvement in bootlegging.
Likewise does Susannah really love Tristan himself? Or is she only attracted to the wild and passionate nature of their relationship?
Harsher in Hindsight: When Susannah meets Isabel II, she cheerfully says "we'll be sisters" - since she intends to marry Samuel. They do end up as sisters in law - when Susannah is married to Alfred and unhappy, while Isabel II is married to Tristan - the one Susannah really loves.
He Really Can Act: A lot of critics noted that this was the role that cemented Brad Pitt's leading man status.
The Woobie: Susannah loses everyone she cares about and ends up in an unhappy marriage. Take note of how happy she is when she first arrives on the ranch. Then remember that she kills herself when she has no reason to live.