My goodness. There are so many things I could say.
I think the adaptation of the book into film was done extremely well. I do remember six odd years ago going to see it at the cinema, I was quite annoyed that certain parts and characters were left out. For example, Tom Bombadil, Glorfindel, Elladan, Elrohir...etc, etc. Also, some of the book's poetry I love was left out.
But as I grew older (I'm 20 now), I've come to realise that they simply couldn't include everything, and many things that they left out are included in small parts if you look closely. For example, at Elessar's coronation, a golden haired elf is standing behind Elrond who I like to think is Glorfindel. And in the Two Towers (in a cinematic deleted scene), Treebeard says some of Tom Bombadil's lines.
So because of this, I think that what Peter Jackson et al. did was extraordinary. The scenery they used in New Zealand is amazing, the balrog was done so well...etc, etc. I could say more

In Jackson's version, one of the main reasons they had Arwen be there to meet Aragorn and rescue Frodo was to give her a bigger part in the film. Main reason being she doesn't have a huge part in the books, despite her importance.
The problem with having Glorfindel included would be having a cast yet another character who only appears in the first half of one film. That combined with the above reason probably just influenced Jackson et al. to leave him out.

Because Legolas is one of the Fellowship. It's not really the Fellowship of 9 to match the Ringwraiths of 9 when one of them is missing.