[Farael]

I believe (and this is pure speculation) that it may be to emphasize the end of an era. The same applies for Aragorn, who also was long lived for the standards of the time (although he had the "excuse" of having strong Numenorean blood).

To me, it makes it all the more bitter-sweet that all these great characters lead wonderful lives, lead their peoples to new heights and yet could not conquer death herself. I know that in Tolkien's world death was not to be a curse but rather a gift and yet...

But going back to my previous point, I think that Gimli's long life is just an example of the "greatness" of the "great" of the Third Age that will never be again.