Karen Carpenter could sing the entries in a phone book and it would be amazing. Her voice is not distinctive -- it's just more or less perfect. It seems completely unstrained while capable of carrying any emotion effortlessly. This song doesn't talk about grand passions or deep promises, but somehow conveys deep comfort that a relationship can provide in the mundane ebb and flow of life.
This was one of Taylor Swift's first big hits. As with many pop stars I find that while later songs may be more sophisticated, their initial simple and direct approach can be winning. Swift doesn't really have the best pipes but her skills are as a lyricist and here she sings a clear and simple tale of required love. This is very country in the sense of being much more story telling than most pop. There's none of the cynicism that is so much a part of Swift's more recent songs. Though the closing refrain... "We were so young when I first saw you..." may be sounding a note of caution about the future.