"The Best Day" from Fearless and "Never Grow Up" from Speak Now sound very similar... because they are actually the same story, "The Best Day" from the point of view of Taylor, and "Never Grow Up" from the point of view of her father.
Isn't "The Best Day" about her mother?
Yes, from Swift's POV.
"Love Story" is written as though Swift didn't know how the play ends... neither did Juliet. For all we know, they both die after the song ends.
At first I thought "Love Story" was just your typical pop teenage love song. I was always disappointed that she used "Romeo" and "Juliet" for the names of two perfect lovers for obvious reasons and I assumed that she simply didn't know her literature... but then I noticed a very subtle change at the bridge. The whole song, she sings to her "Romeo" directly, referring to him as "you". At the bridge though, the precise line is "Is this in my head? I don't know what to think/he knelt to the ground and pulled out a ring..." In other words, the entire climax of the song may very well entirely be in her head, the wistful dreaming of a heartbroken girl, which fits the Romeo and Juliet imagery perfectly. (Also makes more sense of the echo at the end.) Now I hear the song as a bittersweet love that never was, rather than the bubblegum pop love that everyone seems to think it is.
Or the switch to the second person means that her really Romeo, the one who proposed, is someone else, and she's sharing the story with the boy who missed his chance. Never thought about it that way before...
If you want to link it back to the actual Romeo and Juliet, the song could be seen as an alternate ending to Romeo and Juliet, with Juliet narrating to Romeo, where Paris proposes while Romeo is in hiding in Merano and Juliet, tired of waiting for her Romeo to return, accepts.
Or you could see it as an alternate ending in which, rather than going the sneaking-around-and-making-stupid-plans route, Romeo came to his senses and, as the boy in the song does, just asked Juliet's father for permission to marry her. After all, when Tybalt brought it to his attention that Romeo & co. had crashed the Capulet party, Old Capulet's reaction was, essentially, "Sure, I hate his parents, but I hear he's a good kid, so leave him alone." If Romeo had asked him for Juliet's hand, he very well may have said yes. Perhaps this is Taylor's version of a Fix-It fic?
While not as terrifying as some of the others on this list, "Fifteen" has a small bit of "Wait a minute..." She sings that when you're fifteen, "you're on your very first date and he's got a car." Innocuous at first, until you realize that in most of the US, you only get a learner's permit at 16, and a license actually allowing you to drive a non-family member at 17 or 18, which means her first date might be a significantly older person taking advantage of a younger girl.
There's also places where you can start learning at 15 and a half and drive on your own 16. So not necessarily creepy.
Also, just because you're not legally supposed to drive someone you're not related to at 16, doesn't mean it doesn't happen.
According to The Other Wiki, she was living in Tennessee when she was fifteen, where you can get a license at 16. So, not necessarily that much older of a guy.