Of course, he had to point out that as the writer, Neil Gaiman, adapted the story for the screen, he seemed to feel that he had to "sex it up." In the poem, Beowulf is, as near as we can tell, celibate. Not so in the movie. [This BTW is one of the biggest differences between literary heroes and movie heroes. Yes, Philip Marlowe does marry Linda Loring, but not until the unfinished last book in the series. No doubt, the reason for this difference is that the movies are more democratic. The masses are not interested in celibate heroes.]
In principle, Jack didn't mind that film-makers changed the poem into a completely different sort of story -- an 3-D thrill ride with oceans of gore and eye-popping spectacle -- indeed, the anonymous Beowulf poet was no doubt also re-imagining yet earlier stories. Being "unfaithful" to your sources is keeping faith with the tradition!
But he did regret that the film makes no use at all of the single greatest feature of the poem: the language! I noticed that they put a number of songs in the film (good idea, that! these people probably sang a lot!), but in every case the lyrics were rhyming verse, not alliterative verse. What for?
He also pointed out that the visual design of the film, though often beautiful (especially Grendel's cave), had almost no connection with the design of Viking-age artifacts that survive. [For an insight into what this means, get Jack's brand-new edition of Seamus Heany's translation of the poem. It is crammed with gorgeous full-page illustrations of period artifacts.] Again, this seems like a terrible waste to me. The film seems more influenced by the design of video games like World of Warcraft than by actual Ango-Saxon culture. Great film-makers take everything that is great in their sources and build on them. The present strategy seems to me like turning your back on a banquet to catch and eat flies (to borrow a simile from H. L. Mencken).
Anyway, though this is not Lord of the Rings or 300, it is worth seeing. I recommend that you only see it in IMAX 3-D, though. Otherwise you will sit there wondering why they keep hurling things (spear-points, rocks, pieces of furniture, dead bodies, monsters) at the camera. And you'll miss half of the fun. More than half, actually.
By the way, I haven't mentioned what I find the movie's greatest virtue because it was something that would probably influence no one else. As the world's only fan of Crispin Glover, I relished his portrayal of Grendel. I would have seen it if only for that!