Oh, I'm so sad that you didn't enjoy the movie more. It's been upwards of two years since I read the book and I do remember enjoying it and thinking that it was a bit of a throw-away read. Considering that I read it immediately after American Gods that's not surprising. Stardust is a fluffy, zero calorie read. Yvain isn't as likeable in book form and most of the characters were reduced, as you pointed out, to mostly 2D archtypes. That said, from what I've read in interviews Gaiman has given on translating the book to film, I don't think that Stardust was really meant to be anything more than that. It really isn't an excercise in anything and there is no "point or purpose". Gaiman has always struck me as a writer who writes for the love of it and just because he has a story to tell and why not tell it to you. What you get from the story is mostly up to you. I don't think that, in general, there is a raison d'etre beyond I have an idea.

So, asking why is maybe futile for this particular writer and book. Turning the book into a movie maybe the same ... Although, since someone had to buy the rights, I'm thinking that maybe they saw the similarities to The Princess Bride and the current popularity of these sort of fantasy films and decided why not. Although, I also think that they took a bit of a risk with Stardust because it had such the chance to be a colossal flop.

Personally, I adored it. It was charming and bright and sweet and just plain fun. I didn't get a chance to write up a review on it and moved on to Waitress instead. The funny thing is that the one thing that EVERYONE seems to love about the film is the thing that I did not care for, and that is Robert DeNiro. He's a great actor and he really did a great job with the part, but I found him on just the wrong side of too campy. It was distracting. Otherwise, I think the movie was absolutely lovely.

All in all good stuff.