Odd plot structure (spoilers)
One thing that's always struck me about this movie is that the central plot conflict is almost entirely resolved about halfway through the film.
The first half deals with how Toula's decisions to go to college and work and romance a non-Greek boy conflict with her family's plans for her. But as soon as Ian decides to get baptized in the Greek Orthodox Church, the basic plot conflict is over: the family has accepted what she's doing. To be sure, there's still some tension that doesn't get totally resolved until the end when her father gives the "We are all fruit" speech and buys them a home, proving his full acceptance of his daughter's marriage. But most of the second half of the movie concerns wedding plans, nothing more.
I remember when I first saw the film in the theater, and throughout this second half I was continually expecting there to be some turning point where Toula decides to call the wedding off, or at least where she gets into some kind of argument with Ian, followed by a make-up scene. I'm so used to that convention in romantic comedies that I was actually surprised this film never went in that direction. I'm glad it didn't; based on what was set up, it would have seemed contrived and unbelievable. But it did give the film a slightly odd structure.
The writers just followed the story where it led, and didn't try to force it into the formula it broadly used. This moved the focus away from the plot and toward the small moments, which I think were crucial to the film's popularity. The movie wasn't about an ugly duckling, or a rocky romance, or a woman defying her family's wishes, but rather about the overall experience of people from two cultural backgrounds coming together.