Curiously, I just watched "Silence in the Library" two nights ago.
I'm sympathetic to both of the views expressed --
Alidar Jarok's view that Donna's nothing special to River, the bit of dialogue was just to hint at Donna's fate at the end of the season; and
OmahaStar's view that the Doctor built up Donna in River's mind. Both views work within the context of the scene where River and Donna converse.
(The interesting thing about the Donna/River scene is how poorly Donna comes off. Donna is as possessive of the Doctor as Rose was in "School Reunion" when she met Sarah Jane. Moffat wrote the abrasive "Runaway Bride" Donna, not the more mellow Donna of the fourth season.)
I personally favor the latter view -- the Doctor who took River to the Singing Towers and cried could just as easily tell River with great sadness about the companions who made the ultimate sacrifice because of traveling with him. I think it fits better with "Journey's End" and the Doctor's feeling that he can't travel with any one any more, that he would be carrying the emotional scars of what he did to Donna for a very long time. (It fits better, too, with "The Time of My Life," which is one of the top five
Doctor Who comics of all time, which shows how the Doctor is Donna's best friend and what he meant to her. I'm parentheticalling this, because some people inexplicably don't count the comics.

) Admittedly, this reading makes more sense if the tenth Doctor were River's Doctor, because the eleventh Doctor seems to carry no psychic pain from the tenth Doctor's era, but based on "The Time of Angels" I've always thought the tenth Doctor had at least one more adventure with River. (The eleventh Doctor seems more familiar with River than a second meeting would indicate.)
But the former view, expressed by
Jarok also works. River could be awed by Donna not because of any sacrifice but because, as she alludes to in "Forest of the Dead" in the speech about old photographs, Donna represents the Doctor's past, a part of the Doctor's life that River doesn't know.
In the end, I think that River's awe of Donna held some
meaning for her, that River knew who Donna was, what Donna did, and why Donna
mattered to the Doctor.