Longer answer: In the Dungeons and Dragons community, there is an old joke that the characters "kill things and take their stuff." Well, Neil Gaiman and Roger Avary managed to kill Beowulf and take his stuff. Then they proceeded to kill Hrothgar, the helm of the Shieldings and take his stuff. Then they killed Grendel and took its stuff. Then, they killed Wealtheow and took her stuff. And so on, and so forth. These two imbeciles with Underwoods (an orangutan could have come up with a more sensitive treatment of one of the seminal pieces of English literature!) completely changed the tone of the poem from a serious heroic epic to just another post-modern round of "no more heroes" buffoonery.
The literary atrocities of Messers Gaiman and Avary upon the source material are as follows: 1. The poem has been dechristianized: On the one hand, it does take out a glaring anachronism (the action of the poem takes place during the Migration Period of the AD 400s-500s, when they would still be following the traditions of the Aesir religion, but the poem was written down in a very Christianized context in England, and the anachronism does add a richness to the language) The only sop to the underpinnings of the source material come in a discussion between two urinating Danes over the relative merits of Christianity and Aesir-worship, and later when Unferth suggests praying to Christ as well as Odin, a suggestion that Hrothgar rejects out of hand (perhaps a reference to the opposite situation in the poem, where the Danes throw off Christianity for a time, hoping that the old gods will smite Grendel where the Christian ones had apparently failed).
2. It is implied very heavily that Beowulf was a liar and braggart in his earlier exploits, including the race with Breca: As with many of the other changes, it seems to be part of a deliberate campaign by Gaiman and Avary to strip away the heroic nature of the source material, turning Beowulf into just another trendy 21st-century flawed anti-hero.
3. The characters often speak with a much more modern speech pattern (see, for instance, Unferth's first confrontation with Beowulf, where he comes off as much more smarmy than in the poem) that is jarring to the ear and that often seems to lead, yet again, into Gaiman and Avary's unspoken goal of de-heroizing, de-mythologizing, and de-bunking the poem.
4. Beowulf does not kill Grendel's Mother: In the poem, it's reported as fact that Beowulf kills her after a ferocious struggle. Nowhere in the poem does it suggest that she seduces him and he lies about killing her. Again, it's the old song of "everything you know is false -- there are no true heroes." 5. Beowulf fighting in the nude: Beowulf does forswear the use of arms in fighting Grendel, but nowhere does it say that he would fight the monster in the all-together, tackle-out (with only strategically-placed objects protecting his modesty). In fighting with Grendel's Mother, it is explicitly stated that he is wearing chain-mail armor and that that saves his life. I'm giving a pass to her appearance, as it's never stated exactly what either she or Grendel actually were supposed to look like.
6. Hrothgar is almost explicitly stated to be Grendel's father: No, no, NO! Nowhere in the poem was any mention of Grendel's father made, least of all it being Hrothgar, whom Grendel's Mother would not have been able to have "known" anyway, as he was a consecrated king, and it is implied that Cain's kin could not go near signs of rightful royalty. (Cain, the first murderer, who is claimed to be the ancestor of Grendel's kind, not "Cain," the not-appearing-in-the-poem whipping boy of Unferth's, that is.) 7. Hrothgar and Wealtheow have no issue, because Wealtheow will not sleep with Hrothgar due to his sleeping with Grendel's Mother, and the subsequent romantic subtext between her and Beowulf: Wrong again. Leaving behind the fact that Wealtheow probably was not as nubile in the poem as in the movie and showed no romantic interest in Beowulf whatsoever, she and Hrothgar had two sons, and as mentioned before Hrothgar had not slept with Grendel's Mother.
8. Hrothgar gives his kingdom over to Beowulf and then commits suicide by jumping off the tower of Heorot: At this point, I walked out of the theater and demanded my money back, as the movie had officially jumped the shark with no hopes of return.
In short, the movie was little more than a parody, a lampoon on a great epic.