So as you can probably guess from my post count, I’m new here. I was hesitant about putting my thoughts down on this subject because it seems people have very strong feelings on the matter, but then decided, “What the heck.” I also apologize for the post being very long - I was collecting my thoughts on the matter last night while waiting for my account to get approved, and I guess I have a lot of thoughts. I don't think I'm violating any terms of use in posting a long post, but someone please correct me if I'm wrong and I'll try to assemble a Cliff's Notes version.
Prince Caspian is a story about the dark night of the soul---about the characters finding faith and believing again after that faith and belief has been tested to its limits. This is as true for the Pevensie children, who have spent a year in a world torn by war only return to Narnia to find that Narnia is in pretty bad shape too, as it is for the Narnians themselves, who have seen their world invaded and dismantled by the Telmarines, with Aslan nowhere to be found. Some characters, like Lucy, have held strongly to their faith. Others, like the three older Pevensie children, have drifted to varying degrees.
In the book, Lewis reflected this by having the children seeing Aslan in gradual steps, with Lucy obviously being the first to see him. This is probably something they could have still done with the film, but there would have been some difficulties. It is probably more difficult to show the differing degrees of faith on-screen by this device, since it’s not often that scenes are shot from the point of view of each character’s eyes, nor, would I imagine, is it easy to do. And having each of the four siblings describing how much of Aslan they could see at any given time would probably get a bit annoying.
Instead, the filmmakers apparently took the approach of reflecting each character’s degree of faith by their conduct. You have Lucy, who still believes, seeing Aslan when the others do not. There’s Susan and Edmund, who still believe deep down, but have had their share of doubts. And of course, there’s Caspian, who plays the role of the newcomer to faith---he’s been educated in the history and the stories, but it’s through the events of the movie that he truly begins to believe.
On the distant end of the disbelief spectrum, you have a character like Nikabrik, who not only does not believe in Aslan, but who embraces his nemesis from the previous film. In the context of Christian allegory, Nikabrik is like someone who, after having their faith sorely tested, finds solace in vices---he is the Narnian equivalent of someone who loses faith and turns to drinking, gambling, crime, etc.
Then there’s Peter, who seems to be a real point of contention for people. My take on it is that the filmmakers portrayed Peter as one who has lost faith in Aslan, and as a result vests all his faith in himself. He represents the people in real life of whom it is often said, “They’ve forgotten where they come from and how they got where they are.” Having lost his faith in Aslan, all Peter remembers now is that he is the High King. He has, as Lucy aptly pointed out, forgotten how he became High King---with Aslan’s help, as it was Aslan who ultimately defeated the White Witch.
BarbarianKing mentioned that Lewis held the High King to higher standards, and I absolutely agree. Not only did Lewis hold Peter to higher standards, presumably Aslan held him to higher standards as well. And for as long as he believed in Aslan and remembered that it was by Aslan’s power that he became High King, Peter lived up to those standards during the Pevensies’ reign in Narnia. But after being torn from that world and returning a year later to find that world completely devastated, with no hint of Aslan trying to set things right, the humility that Peter’s faith once endowed upon him has now given way to pride. Peter clings to the title of High King because that’s all he has now, in his mind. He doesn’t think they can count on Aslan, who has been absent for 1300 years, so now they have to count on him. In real-world terms, he has replaced faith in God with faith in his own successes, while forgetting that it was through his faith in God that he succeeded in the first place.
Of course, it’s only human to falter when we’re held up to the highest standards, such as the standards that Lewis and Aslan imposed on Peter. If that were the end of the story, then, as BarbarianKing said, it would be terribly disappointing. But that’s only half of the equation---we don’t look up to people simply because they’re human. We look up to people who, when they fail---because of their imperfect humanity---to meet the standards that they’re held to, are capable of acknowledging their shortcomings and improving themselves. That’s the aspect of Peter’s story in the film that we’re supposed to look to---not the fact that he’s initially self-absorbed and a bit bratty, but the fact that when his pride led to disaster for the Narnians, he was able to admit that Lucy was right and place his trust and faith in Aslan once more.
I guess the point of this long rambling post is that what the filmmakers did was different in execution, but still aimed at preserving Lewis’ idea. I don’t think their portrayal of Peter has made him a less admirable character in the end, because ultimately the story in Prince Caspian isn’t just about having faith, as Lucy did all along; it’s also about finding your way back when you’ve strayed, as Peter, Edmund, and Susan had to do. As for how it will affect Dawn Treader… I’m not sure. In part because, as you may have gleamed from my long-winded writing, I tended to focus on the fact that Peter was able to come back to faith after stumbling, rather than his actual behavior when he was off the path. I don’t see a problem being critical of Eustace’s behavior, because people should rightfully be critical of Peter’s behavior as well---so long as they cut him some slack once he recognizes the error of his ways and once again becomes the High King everyone expects him to be. Perhaps some of the characters should have been a bit more critical of him during his bad Peter phase, thus making it more consistent for people to be critical of Eustace, but I think it was also hard to find characters from whom such response would be in-character---the centaurs, Reepicheep, and Edmund seemingly were all scripted to be loyal to the end, and thus would not be ideal to criticize Peter. Susan shot him a few disapproving looks and questioned his motives in the castle raid, and Nikabrik was clearly disdainful of him after the castle fiasco, but Nikabrik was also so far down the road to being bad that his criticism of Peter’s behavior lost most of its credibility by virtue of the character being a near-villain. The fact that Peter’s title of High King shielded him from much of the criticism from his former subjects would probably serve to temper any disparity one might find in criticizing Eustace but not Peter; people are naturally less inclined to criticize someone who holds a position of respect and honor, even when they’re acting poorly. I’m not saying it’s the right thing for people to do, but it explains any inconsistency in the treatment of Eustace vs. the treatment of Peter in a way that just about everyone understands.
As for Susan/Caspian (I know, long-winded)… not a big deal. It was just a kiss. Maybe it’d be tougher to sell to little kids, since when you’re a little kid, even holding hands is a big deal and kissing means you might as well be married, but for most people, I think it’d require at most a minute of dialogue, if it’s given any screen time at all.