There have been many reviews of Stardust comparing it to The Princess Bride. While I think the comparison is valid, it is less because the movies are terribly alike, and more because they are of the same genre. Off the top of my head, they are the only two movies I can think of in that genre. For the sake of continuing to talk about this genre, I'm going to start refering to it as Fairy Tale, but there is a distinct possibility that that name will turn out to be a misnomer as more movies are made in the genre.
Before defining what this genre is, I'm going to explore what other genres have in common that differentiates this one from them, because I've begged the question by naming the new genre Fairy Tale. Surely, I hear you cry, there are tons of movies in the Fairy Tale genre. I say that there are tons of movies based on fairy tales that have been made in other genres. Whenever Hollywood makes a movie from an existing story, they do so in one of the genres they are comfortable with. It is a pretty solid rule that when a story is made into a movie, some elements will have to be dropped to make it fit into a two hour movie. Which elements are cut will depend on the genre in which the movie is being made. If we're making a romance, we'll tend to drop action sequences. If we're making a thriller, we tend to drop comic interludes. And so forth.
What distinguishes Stardust and The Princess Bride is that they are made in the genre of Fairy Tale, with all of their elements intact. Cuts have certainly been made, but they are not categorical cuts, but primarily ones of duration and detail. They contain all of the elements of their respective base stories, not sacrificing them for a genre. It is entirely possible that other movies will be made with all of their elements included, if truncated. If some of them fall out side the fairy tale classification, I'll have to change the name of the genre, until then, I'm sticking with Fairy Tale.
I think that being part of this genre is a large part of what will help these movies endure as classics. Being fairy tales makes them timeless, but being complete will make them stable.