The purpose of everyone being there. In the film, it is implied that they were arriving on some sort of schedule, or had been summoned to a meeting for the purpose of discussing the ring. In the book, they have all happened there for their own reasons, many having to do with the growing darkness in the east, but were troubled by their own affairs and had all come there at the same time by chance. Though most of their troubles are related to Frodo's mission, they don't know it yet when the council begins.
Why do you suppose Bilbo was not seated at the council? - even if Jackson didn't want to give him as much of a part there, for brevity, I wonder why he didn't sit him in Figwits chair. (would have saved us from the figwit business we are left with)
First off, in the book, the Council was a much more spontaneous event. All the particapants just showed up, for various reasons (some of them supernaturally summonsed, others wanting advice) - and they've got problems that don't seem, at first, to relate to the Ring.
Of course, the book council was longer - all those stories! I'd been hoping they'd leave in at least the Dwarves' story, about the messenger coming to Dain - anything for a glimpse of the Lonely Mountain! But no... oh well, I'm sure it would just have bored the non-fans to sit through.
Boromir dominates the Council a lot more in the movie. He's like that annoying guy in the room who doesn't understand that everyone's giving him the Glare of Death and hoping he'll shut up. Even Elrond doesn't want to put up with him.
And of course in the book, the Fellowship gets mostly put together after the council. Those are the biggest details - the movie differs mostly in leaving things out rather than changing them. Not bad at all. Not at all like the Council scene in Bakshi... *shudder*
Probably because Bilbo's only purpose at the Council was to tell his part of the "Ring" story and in the movie, that was told in the prologue.