What spoiled it. (SPOILERS!)
I think the adultery spoiled it.
The challenge with Marie Antoinette is to make palatable a story with a horrible ending.
Valkyrie (2008) did a good job of that (though not good enough to make it a big hit) by presenting the protagonist starkly as good. Maybe flawed in some ways, a little headstrong, whatever. But, by his own standards, which we are clearly meant to share, good. That way, there's some redemption. It's like: "Yeah we lost, but we were the good guys."
I thought that Marie Antoinette (2006) was shaping up like that, and I was very happy most of the way through. We could see that Marie Antoinette wanted to be a good wife and a mother, and she had to struggle a little to achieve that, but it worked out.
Marie Antoinette had a standard of virtue, which she applied maybe a little uncharitably to Madame du Barry, but which gave her a clear benchmark too: sexual chastity is fundamental. So my expectation was: she's tempted, she resists, the ending is horrible and deadly, but regardless of how irrelevant history might find her little victory, it was hers. Flighty and unfortunate, but faithful. Yay Marie Antoinette.
Instead of which, after having finally achieved a thoroughly good and fruitful marriage - cut to the eye-candy soft porn with the foreign soldier.
OK, the awful end arrives, though the curtains are mercifully closed. So: she was punished by history for being a tramp? Obviously not, she and her family were crushed by historical forces irrelevant to all this.
Then what is the point? There is no point. There were lots of nice pictures of pretty dresses and strawberries and such, and there was enjoyably anachronistic music, and ... that's about it. No story worth a darn. What a disappointment.