The film adds the possibility that Susan's eventual denial of Narnia is due to the romance that could never happen with Caspian.
In contrast to her stance as Queen Susan the Gentle, do Susan's rather aggressive tendencies on the battlefield come from somewhere else? She ruled as a Queen for fifteen years in what appears to be a progressive society - and then had to return to 1940s England where women were still in Stay in the Kitchen roles. That's bound to make her long for the power and freedom she once had. Likewise women did serve roles in the war, but Susan as a teenage girl probably wouldn't be able to - thus frustrating her even more.
Big-Lipped Alligator Moment: Susan and Lucy partying with Bacchus while the boys rescue Caspian. It has no impact on the plot, the mythology crossover is never explained, and it's never mentioned again.
There's also Susan becoming an Action Girl. In the book her adult self is described as not liking fighting "like an ordinary grown up lady", which is why she was named Queen Susan the Gentle. On the other hand both Anna Popplewell and Andrew Adamson said "if she was going to Stay in the Kitchen she should have been given a slab of bread and some butter, and not a bow and arrow."
Die for Our Ship: The Susan/Caspian shippers, despite the fact that there's absolutely no mention of it in the books, they never see each other again, and Caspian marries Ramandu's daughter at the end of Voyage of the Dawn Treader. Curse you Andrew Adamson for including that kiss.
Love to Hate: Sergio Castellitto's take on King Miraz, making him a very effective and ruthless villain - with the occasional Pet the Dog moment to humanise him. Likewise he also gets some good Deadpan Snarker moments.
One-Scene Wonder: Tilda Swinton returns as Jadis for one scene, as an expansion of a point in the book where they consider resurrecting her.
Sophomore Slump: Sort of. This was the second book published (though the fourth chronologically), and it's commonly regarded as the weakest. The film is a mixed bag. Some felt it was a step down, others loved it.
Strangled by the Red String: The romance between Susan and Caspian is accused of this. There was none in the book and it was Doomed by Canon anyway. The whole thing is very chaste and never progresses beyond simple flirtation—culminating in a small kiss at the end. Word of God is that the romance was more detailed in the original script, but cut down to a more chaste thing because they wanted to avoid it taking over the movie.
They Changed It, Now It Sucks!: This film provoked this more than the first. Due to the book's iffy pacingnote The Pevensies arrive in Narnia, meet Trumpkin and then he tells them everything that had happened up until then (including Miraz storming Aslan's How), a lot of things had to be changed. Things that were especially disliked were the castle raid and the romance between Susan and Caspian.
Unintentionally Sympathetic: Peter and Caspian's fight is meant to show that both of them have messed up majorly. Except Caspian abandoned his post in the middle of a covert military operation just to get some revenge - which he didn't act upon. The castle raid failing was far more Caspian's fault than Peter's - who was more trying to salvage the mess that Caspian made.
What an Idiot!: The assault on the castle in Prince Caspian (which did not happen in the book) gave huge helpings of this to Peter. To be fair, everyone from Caspian to Lucy calls him out on it. Caspian also deserves a share of this one because it was his fault in the first place that they lost the element of surprise; he went into the castle with his own agenda and despite the obvious results of him changing the plan in the middle of a Stealth-Based Mission he shows no responsibility or remorse for his actions. To maintain order among the Narnian army, Peter has no choice but to let it go and share the blame.