I'd have loved to see this fox philosophising about Sam and Frodo from FoTR laughing ok, would've looked stupid in a movie, but he was so cute...
And of course the story Glorfindel tells during the council, about his surviving the downfall of Gondolin =)
And this wonderful scene in the Glittering Caves when Tolkien describes how ... awww there's no way to find any other words for it:
And plink! a silver drop falls, and the round wrinkles in the glass make all the towers bend and waver like weeds and corals in a grotto of the sea.
Ya, Glorfindel at the council telling about Gondolin would have been swell, but think about all the people that didn't read the Silmarillon and only checked out the movie for fun. They wouldn't have understood it. For them to like it, PJ would have had to make a scene of battle at Gondolin and, let's face it, he didn't have the same budget as he had in the last 2 movies.