I find it difficult to describe what it is I love about this series. I suppose it's the way the characters and their relationships are developed. The type of situation I'd expect in a shoujo is to have the anxiety-prone female protagonist and the love interest she doesn't know what to think of. Often you see many sides to the boy, and just one side of the girl, and the story consists of the girl trying to make sense of the boy and their relationship.
But in My Girl, you see different sides to the protagonist as well. Ahjin in all forms has a crush on Dohoon. Ahjin, as Ahjin, is self-conscious about her boyish look and how Dohoon sees her as a little brother. Ahjin, as Jinny, behaves more confidently (she acts "in her element") IMO, and isn't as inhibited by Dohoon's perceptions of her (since she's aware he doesn't know she's Ahjin, she can be more direct about her reactions to his behavior). Dohoon has a perception of who Ahjin is versus who Jinny is and acts uniquely toward each of them.
So in this way you have different sides of the characters in each situation, rather than just seeing the same type of behavior every time two characters meet up with the occasional small development which causes them to see each other a bit differently. This is not to say that the story has a whole ton of depth of character, it's certainly no Kare Kano, and I will say that there are definitely is resemblance in structure and plot/character elements to other shoujo I've read, but the way the plot ties in with character development is excellent, and in no respects is it substandard. This is an enjoyable, refreshing, fairly creative and unshallow story, and I'm not surprised it received a lot of 10's, fans of shoujo who are attracted to a manga like this will notice and appreciate that IMO.