I didn't find Susan to be annoying in The Lion, The Witch, and The Wardrobe. She did try to be reasonable and logical about things, but reason and logic aren't bad or annoying things. Indeed people were created with reason and logic and in that sense we can say that reason and logic are good. We even see how the Professor in his conversation with Peter and Susan does not condemn logic and reason as bad (it would be a strange position for a professor to take that stance) but he does try to guide Peter and Susan to what is the correct logical and reasonable decision. That Lucy because she has a history of being more truthful than Edmund is telling the truth, where Edmund is lying, and that Lucy has not gone mad as the saying goes as can be attested by the rest of Lucy's conduct.
I also got the vibe that she is sort of trying to be like a mother to Edmund and Lucy since they are separated from their own mother in a similar way that I got the impression that Peter is trying to be a father figure to Edmund and Lucy with their own father off fighting in World War II. That's some big shoes for Peter and Susan to try to fill. Some big responsibilities and roles to take on in a family.
I can understand why Lucy and Edmund might be irritated by Susan sometimes and why even Peter could sometimes be irritated by her efforts to be reasonable and logical at some moments in the film, but as an audience member, I could get where Susan was coming from and sympathize with her. I actually could sympathize with all four of the Pevensie children in this film, so I think the movie did a good job with that.
I saw Susan as trying to be a mother to her younger siblings. In that way, she truly seemed to be "Susan the Gentle" to me before Aslan ever crowned her queen.
