Fridge Logic: If the girl is SO sensitive that even a pea through 20 mattresses will leave her bruised in the morning, how did she get far enough in the storm to even make it to the prince's castle in the first place? Shouldn't she have arrived a broken and bloody half-dead mess from the rain? (For that matter, how is she gonna have kids?)
It didn't leave her bruised, it was just that she could feel it period. Doesn't help the childbirth fact, but so long as it was just a moderate storm, the kind you wanted not to spend the night in, it shouldn't have been terribly damaging.
No one said she must be a good person. The task is made for Spoiled Brats.
The Live-Action TV version featured in Faerie Tale Theatre attempted to "civilize" the morning conversation. She was being polite at first and denied the sleepless night when questioned, until challenged to tell the truth, and then she spilled the beans in a more woobieish fashion (especially considering that this version also showed exactly what she went through that night).
Expanding on this point, some later adaptations of the story have the princess be polite and deny that there was a problem the first one or two nights. Then she either admits that there's something wrong with her arrangements, or the prince and his family come in to actually see the troubles when she fails to come in for breakfast.
One possible interpretation is that it is better to be honest. Though let's face it; the moral is "it is really good to be a princess." Which sucks for the rest of us.
The main moral of the story is that "Appearances Can Be Deceiving", when the princess first arrives at the castle, she looks like anything but a Princess. The narrator describes how the water ran down from her hair and clothes; it ran down into the toes of her shoes and out again at the heels. This is not exactly the elegance that we usually associate with royalty. Naturally, the queen is suspicious and wants some form of proof that this stranger is the 'real princess' she claims to be. After passing the sensitivity test, it's revealed that the girl is who she says she is and the Queen herself learns a valuable lesson on appearances.
Also in the version I grew up with, when the princess is questioned the following morning, it's clear from her appearance that she didn't get any sleep, but she politely says she slept fine until the Queen presses the issue enough to get the Princess to admit that she had difficulty sleeping.
Tropes from the film
Angst? What Angst?: Although Heath is briefly shocked when he finds out about Laird swapping Daria and Hildegard, he gets over it rather quickly. At the end of the film, he doesn't seem too upset that the girl he raised from infancy to adulthood turned out to be a fraud who was conspiring against him, and that his own daughter is practically a stranger to him.
Fashion-Victim Villain: The shoes Laird wanted to wear to his coronation are hideous, and he vowed not to take them off until his vengeance was complete.
Idiot Plot: The fact that infant Daria was able to sit up by herself means that she was at least 4-7 months old, and that she would've had a distinct facial structure, and eye color. Laird's plan to swap princesses would have failed immediately if anyone had noticed the different-colored eyes, lack of birth mark, or the fact that Hildegard doesn't look remotely similar to either Heath or the queen.
Narm: By the truckload. Standout scenes include the awful CG effects when the stained glass figures come to life and at the end when Daria runs into Heath's arms, he spins her around, and then sparkles come out of nowhere as a scene transition.
Rooting for the Empire: Laird's motive for wanting to take over are pretty understandable, him having previously been destined for the throne before it was given to Heath.
They Copied It, So It Sucks!: The entire plot is lifted from Harold MacGrath's "The Goose Girl", which is ironically based on another fairy tale entirely, albeit the film is a bit more Disneyfied. For example, Hildegard is not a villain in the original book, which makes sense considering the whole being a baby when the switch occured.
During the beginning of the film, Helsa shows compassion towards her infant niece, and mentions how similar both her and Daria are (both not having mothers). She also shows hesitance over going through with Laird's plan and asks that he not harm Daria. After this scene, she's just as evil and irredeemable as her husband, and any hidden depths she might've had were forgotten.
Hildegard turns out to be just as evil and heartless as both her biological parents. Even though Laird uses it as a way to get Rollo to sympathize with her, his (false) explanation for Hildegard being so cruel is because she has to live up to everyone's expectation of being just like her "mother", would've actually worked as a real reason for her attitude. Her "father" Heath constantly tries to get her to act similar to her "mother", and even told his infant daughter he hoped she'd grow up to be just like her mother, so she would've had this pressure on her since she was a baby. There's also the fact that she was raised by both Helsa and Heath, with one teaching her to be kind and selfless and another teaching her to be vain and selfish. This could've lead to her being more a more conflicted person, trying to figure out right and wrong.
The Woobie: Daria has it pretty hard throughout the film. She's taken from her loving father and raised by mean, lazy pig farmers who she has to take care of, she does all the chores on the farm, sleeps in a chicken coop, has the guy she's in love with ditch her, an angry mob tries to kill her, everyone ostracizes her when she goes back to town, and then her uncle and cousin try to kill her.