I couldn't disagree more on what a game essentially is, or what the role of critics should be.
Being a critic isn't being a consumer guide, which would be an utterly pointless exercise when called to judge a creative product, to which there are likely to be as many angles as there are people. If you simply want to know what everybody might like, then we'd be better off replacing critics with focus groups. No, the role of the critic is to challenge the reader with his opinion, preferably based on a wide range of references, and a deeper than average understanding of the subject at hand.
Fun is again purely subjective. Take almost any given comedian, and I guarantee you that within 10 people, you'll find great disparity between those who find him funny, and those who don't. We enjoy different things, and that's exactly what good, honest criticism is meant to represent, although with the maddening rush to consensus that almost always arises between game critics, I wouldn't blame any gamer for never realizing that.
Your claim that writing should simply be ignored because games aren't movies simply doesn't hold up. If something contains dialogue, if it contains acting, if it contains a plot, then it's every bit as valid to challenge that dialogue and that plot and that acting, as it is with any other medium that contains the same, whether it's a book or a movie. A movie is technically a visual medium, but you'd never hear someone saying that writing isn't important, simply because someone added moving pictures to the story. In the same way, writing doesn't become less important, simply because someone lets you control the characters of that story.
You're not moving chesspieces around in black and white squares, you're moving people around, in what's supposed to be a living, believeable place. Their circumstances, actions and behaviour all need to own that.
I don't see why you think we disagree on the gameplay. I pretty much totally agree with what you describe above. The mechanics are great when they're properly applied, and sneaking around is often hugely rewarding.
But what I'm saying is, when you give something a 9, pronouncing it 10% short of the highest perfection, then you need to be able to back that up, and quite arguably there are major issues with the gameplay as well. The side missions quickly descend into pointless fetch quests, for which the developers are still every bit as accountable, and the AI doesn't exactly provide you with much in the way of a challenge.
Overall I think it's acceptable, and my problem isn't mainly with the mechanics, but 10% short of the highest perfection, it is simply not.
As I describe above, I think it goes from moments of utter greatness, to moments of almost complete shame, and that's the reason why I'd give it a 5. Not because it's middle of the road, but because it's keeps mixing the truly great, and the really terrible.