The Ron-Hermione romance doesn't work on film

I've been trying to figure out why the Hermione-Ron romance is so much more believable to me in the novels than in the films.

I think one reason is that, in the novels, Rowling takes a lot more time to develop their relationship, to show them interacting and drawing closer to each other, so that the major events in their romance (like Hermione crying when Ron is dating Lavender or the big kiss in The Deathly Hallows) don't seem to come so much out of the blue.

I'm thinking, for example, of the many scenes in The Goblet of Fire where Ron reacts with jealousy to Hermione's relationship with Victor Krum. Those scenes didn't make it into the movie.

But I think the most significant reason is physical: the Hermione and Ron, as described by J.K. in the books, fit together. Hermione is a bookish girl with bottle-brush hair and too-big teeth. Ron is a tall, gawky red-head with a big nose. They seem right for each other.

Emma Watson and Rupert Grint, however, don't look well together. Watson is just too classically pretty by the end of the series whereas Grint's handsomeness is more lunky and sheepish. You can see Watson's Hermione ending up with someone like Cedric Diggory (had he lived), an athlete with a ready smile and movie-star looks.

In reality, it's probably a combination of the two factors but the movie relationship just doesn't work for me like the book one does.