Please read Ivan's great post on Miracle on 34th Street (the original, John-Payne-not-John-Hughes version). He rightly points out how "there is a thin coating of cynicism surrounding Miracle, and how the characters in the movie are motivated to do the right thing—for the wrong reasons."
That's one of the things that makes Miracle so much more than another sappy true-meaning-of-Christmas movie; while it pushes the message about the need to have faith and be irrational, it's also very clear-eyed about the fact that people act in rational, self-interested ways. Many of the remakes have forced the characters to become infected with the sentimentality of the premise, turning them into a bunch of mushy fools, like the John Hughes version and the Broadway flop Here's Love (a musical from Meredith Willson, creator of The Music Man, who was so totally out of ideas that he put in his old hit "It's Beginning to Look A Lot Like Christmas under a different name). But the original Miracle plays perfectly well as a romantic comedy about a crazy old coot who brings two people together; it's not trying to inflate itself and it's not trying to drop an emotional anvil on us. Even the ending is played as a goofy joke; it sends us out laughing.
And it also benefits from incorporating some of the neo-realist feel that Fox movies had in the late '40s, by going outside the studio and shooting in New York and inside Macy's. If it had been made a few years earlier it would have been shot entirely in the studio, and that would have made it feel more like a sappy fantasy.
George Seaton, though... he had a good career, but every time I look at his filmography I can't help thinking it should have been better. After a decade as a very successful screenwriter for comedy (A Day at the Races) and sentimental drama (The Song of Bernadette), he was allowed to direct his script for a Betty Grable musical, and it became perhaps her best film, Diamond Horseshoe. He formed a partnership with Fox producer William Perlberg, and they made some good comedies, dramas and musicals for Fox (The Shocking Miss Pilgrim, another Grable musical, has a Gershwin score entirely created from Gershwin's trunk material, with new lyrics by Ira Gershwin). But apart from Miracle, his movies fell into the category of pleasant things to watch on TV on a Sunday afternoon, as opposed to classics. And after he left Fox, the movies were still short of classic status but longer and less fun (The Country Girl, Teacher's Pet). In 1970, when most of his contemporaries were forced into semi-retirement, he had a huge hit by writing and directing Airport -- a movie that felt like it could have been made 20 years earlier -- and then got forced into semi-retirement anyway.
It's enough that he created one classic like Miracle; most directors don't even do that. But given his talent as a writer and his obvious skill at directing actors (many actors got Oscars or Oscar nominations for working with him), I wonder why a lot of his movies feel like they're missing something.