I thought Grendel was a great and creepy re imagining. It seems silly to state you wouldn't have imagined him like that, when it quite obvious they came up with another take on Grendel as they did the Beowulf story, he wasn't supposed to be like the poem in looks or really in actions (Besides killing people). Personally I think this is going to be one of those movies where people try to look for something deep trash it, but people who are willing to have fun, without being always concerned about something "deep" (ask me so many of these much heralded "deep" films are almost as shallow as Beowulf) and get a decent story will enjoy it. Yeah you'll have your pompous butts like Ebert dislike it (Mr. Video games aren't art), but people that appreciate a good time will enjoy it. Also people who enjoy the original source but refuse to like something that changes the original source, will more than likely dislike it as well. I know for movies, I try to separate the source material from the movie, it makes a much more enjoyable experience when you come into thinking of it as something different (In fact when dealing with things like comics I appreciate something that isn't line by line like the comic).
This is probably going to be blasphemy, but the basic plot of Beowulf is probably deeper in this movie (Note the word basic plot), because in the poem it goes like:
King needs help, Beowulf kills Grendel, Beowulf kills mother, Beowulf kills dragon sacrificing himself. The end, at least this movie gave some depth to character of Beowulf, showing he is a flawed man and even the most mighty can fall into temptation, but the true heroes redeem themselves and don't wallow in their pity like Hrogar did, which IMO is a true hero. If I recall correctly, the Beowulf from the poem was quite one dimensional, he was pretty much your perfect hero. At least this version of Beowulf is complex, and yes, more human than the one from the poem.