A few thoughts:
First, I was very impressed with the game's length. It took me 18.5 hours, which is almost unheard of for non-RPG single player games these days. It never dragged, though, and I found the pacing to be suitable and the challenge to be well balanced. Yes, the A.I. sometimes did stupid things and yes, at a certain point some elements got repetitive (and I agree with Niode that the puzzles were all easy), but when those repetitive elements are fairly fun, I didn't mind.
I loved that the majority of the encounters were tense and, because I don't generally care for stealth too much, provided a pattern of intensity that moved from "strategic planning" to "nervous exacting" before things would (often, but not always) shift to "frantic aggression" and sometimes "desperate actions". I had just as much fun sneaking up on a clicker and shivving them as I did jumping over logs in the snow while wielding a hatchet like a maniacal Nicholson from The Shining.
As for the story...
The game is certainly one of the most gorgeous games of the generation and will likely always be cited as one of the best technical accomplishments of the generation. It offers a memorable world and (eventually) a story that ranks amongst the best of the genre this generation, though some will likely hate it for the storytelling choices it makes. The gameplay offers something that's bordering on novelty/innovation, but it can be inconsistent.
So...I don't think I'd be able to hold it against somebody if they DID think this was the game of the generation - I think the argument is there to be made and supported when evaluated by most of the criteria we tend to judge games on. But there's also enough there that someone may see as a problem to suggest that it is simply one of the better titles released on its system/in its genre/in this year/etc.