I have always wondered if Jane Austen meant for the line about Marianne marrying Brandon "with no sentiment superior to strong esteem and lively friendship" to be taken literally. Although we do know that Marianne comes to love Brandon with her whole heart, perhaps the feelings she had for him before the marriage were a growing love that she simply did not recognize or fully understand, as she had only the memory of her intense and passionate, but ultimately immature and fleeting, feelings for Willoughby to compare them with. IMO, the calmer, more respectful, and ultimately deeper feelings that she developed for Brandon would naturally seem less exciting than her "first love," at least for a while. The alternative, of course, is that Austen intended this passage to be taken at face value. I can understand and appreciate either interpretation, and neither of them changes how I feel about Marianne and Brandon's relationship in the slightest.

I can also appreciate when adaptations try to stay faithful to an author's intent, and I have often admitted that, as much as I enjoy it, S&S3 Marianne's dialogue that she loves Brandon and has agreed to marry him comes a bit too early in the story and is therefore not entirely faithful to Austen. However, S&S2 isn't very faithful in presenting the Marianne/Brandon relationship, either, because it does not show Marianne realizing that Willoughby was no good for her, and it shows Willoughby at the end, sitting on a white horse and watching the wedding with a pensive look on his face. White horses (and, in Westerns, white hats) are generally a shorthand for "hero" in many films, and Willoughby's sad, longing expression just provokes sympathetic feelings in viewers. S&S2 makes Willoughby out to be almost a sort of tragic hero who passionately loved Marianne and made an unfortunate mistake, while Marianne chooses security (with Brandon) over passionate love (with Willoughby). Personally, I find S&S2's interpretation far more disturbing than any changes that S&S3 made to the story. I certainly don't expect you or anyone else to agree with all of my opinions, though. That would be dreadfully boring. ;)