Firstly, nothing in the entire movie is anywhere near as frightening as Tom Hanks was in The Polar Express, the last film Zemeckis directed, also released as An IMAX 3D experience. Not even Ray Winston's Beowulf, who seems unable of passing up any opportunity to take his clothes off.
The hugely distorted swords and arrows, which I guess are designed to leer at 3D audiences, are only comparatively weird. There are a bunch of tracking shots which don't work in 2D: they interrupt the action with moments where you feel like you might be politely viewing someone's boring nature photos. The entire cast, with the notable exception of John Malkovich's Unferth, seem prone to unpredictable bouts of Welsh(ish) accent. The whole thing is also filmed using motion capture: At times, if feels like your watching an adult version of Shrek -especially during the sequence when a nudey Beowulf's crown jewels are luckily covered by a range of weapons and strategically placed people. Beowulf standing on the table during his first encounter with the monster Grendel, backlit with flame and only a probably 3D sword for modesty is the films laugh out loud moment.
The story kind of sticks to the central elements of the epic poem - but anyone expecting a The Passion of The Christ type approach to the Old English epic is likely to be disappointed. Beowulfs three enemies - Grendel, Grendel's mum, and a dragon - are all present and correct. The movie most obviously diverts from the original in it's rewriting of Grendel's mum, whose transformation from aggrieved mother hell bent on vengeance to hot gold painted naked dragon lady who likes to sleep with heroes has the obvious advantage of allowing someone who watched Weird Science too much the opportunity to digitally realise their Angelina Jolie fantasies. Grendel's mum subsequently turns into a Viking themed Pirelli calendar, no doubt helping along the box office receipts significantly.
Crispin Glover (who previosly worked with Zemickis on Back to the Future I & II as Michael J. Fox's dad) plays the most interesting character - Grendel, a monster with overly sensitive hearing and inconsiderate neighbours. Despite his taste for human flesh, Grendel ends up being the films only sympathetic and complex character, his tragedy all the sadder for his mum's indifference toward his death, and the comparative boringness of everyone else.
You also get three kick-ass action sequences.