Ok, when it's all said and done, I give it a C+. I don't know which, really. I'll give my reasons.
The script was, indeed, Scripturally based, but the passages lifted from Scripture did not mesh well with the rest of the script. I suppose I'm a bit of a purist about this -- I think Peter Jackson's screenwriters did a superb job on the Lord of the Rings movies. There were also lines omitted for no good reason. One example is Zechariah's canticle upon the naming of his son: it was completely omitted. I seem to recall some of the words or phrases used elsewhere by Elizabeth, while John was still in the womb, but the impact is different. The Magnificat is spoken at the end of the film, but not in its entirety.
[Update: The exact use of Scriptural speech is, in my opinion, an all-or-nothing endeavor. The movie failed because it injected Scriptural passages (like the "Blessed are you among women" of Elizabeth, the Benedicta) without trying to conform the rest of the script of the movie to that kind of language. Simply put, it seemed like Elizabeth was "talking crazy talk" instead of being inspired by the Spirit. I would not have minded if they conformed the Scriptural words to the script of the rest of the movie.]
Other omissions I had hoped to see included are centered around the presentation in the temple (Luke 2:22-38). The Nunc Dimittis (the canticle of Simeon), Simeon's grim blessing to Mary, Anna the prophetess... all missing. This leads me to another issue: the timeline.
In reconciling the Luke narrative and the Matthew narrative, if Jesus was circumcised on the eighth day and presented in the temple after the forty days necessary for Mary's purification, I would have expected that the visit of the magi and the subsequent flight to Egypt took place after that. Matthew 2:11 mentions a house, not a manger. Then again, I'm not a Bible historian, so I don't know what an "official" reconciled timeline of the nativity narratives would be.
This movie also suffers from "translationism". The name of the infant is called "Jesus" instead of Yeshua (or Yehoshua) which is grating amid names like Zechariah. Mary should be called "Miriam" (or something like that). They speak in Aramaic occasionally (when saying particular prayers or greetings) and then go right back into English. Now I recognize the worth of authentic language found in The Passion.
The soundtrack was a little off at times -- meaning, the music didn't really fit the scene. There were several pieces of music based on Christmas songs that just didn't click.
Finally, and I will admit this is a very tricky matter to handle properly, the representation of the angel Gabriel is weird. He seems to appear and disappear with the presence of a dove (or some other bird, but I'm assuming it's a dove), but this has the unwanted effect of making angels transforming animals or something along those lines. I'm not sure exactly how I would do it, but the movie's handling of it seemed too weird.
Despite these complaints, I did enjoy the movie, it just didn't grab hold of me the way I'd have liked it to.