Jonathan D. makes good points, but there are several matters to mention in reply.

Galadriel did indeed give up her kingdom and life, albeit in a different fashion than did Finrod Felagund.  Galadriel "allowed" the One Ring to be destroyed (remember she refused Frodo's offer), knowing that her own powers would be diminished, and that she would have to "go west." She and her ring essentially created Lorien.  She abandoned her kingdom Lorien (which presumably faded), and travelled beyond the circles of the world.  Okay, this is not precisely the same as how Finrod met his fate, but the point is, it supports the theory that the Aragorn-Arwen story was the commercial version of the Beren-Luthien tale.

Beren was given the task of recovering a Silmarill, by a father overcome by lust for the gem.  An impossible task, he figures it will kill Beren to try.  A wiser Elrond, not overcome by lust for the ring, instead imposes the geas on Aragorn of uniting the North and South kingdoms.  Not precisely the same? Of course not, but once again, it's comparable.

As to the idea of replacing Legolas in the Fellowship with Arwen, well, as I wrote previously, some variation from the book may have to occur.  I agree that elf-maidens did not go to war, but Luthien obviously was a special case.  She escaped from dad, defeated Sauron, saved Beren, knocked out Morgoth, then was the only one ever to talk her way out of Mandos. 

If Arwen is thought to resemble Luthien, then maybe the she goes against the rules too.

Someone's favorite character (such as Legolas) may have to be cut in order to squeeze in all the action.  The slowly developing friendship between Legolas and Gimli may be interesting to follow in a book, but may just clutter up a movie which may be difficult to follow even in a simplified form.

Making the movie as a complete documentary version of the Lord of the Rings, staying true to every micro-detail in the books, would simply not work.  The movie would be a disaster, and neither you nor I nor the movie-going public will watch.

For my part, I would be saddened if the character cut from the movie is Tom Bombadil, and if the sequence of Old Man Willow, Bombadil, Lady Goldberry (good cameo role for gorgeous starlet), and Barrow-wights is cut.