MSG4 has to be taken in context, and no not the, "PS3 needed a big exclusive" sort of thing.
You have to consider the progression of the Metal Gear games, which contain many similarities to other games but are in game play style, their own genre. A lot of things came together perfectly with MGS4-
1. The translation of the game from an pseudo- top down style to a full blown 3D game was something that for most games would seem rather late. The complexity of the game itself made it impossible on earlier hardware generations. The difference however allowed for a far more enjoyable combat, a function that was in earlier games, cumbersome and slower paced.
Bringing the Metal Gear gameplay into full blown 3D improved the mechanics, making it not only fun in a tactical sense, but the combat as well - but at the same time, it increased the complexity of the stealth game play, allowing for players to react better.
In the context of the "Metal Gear" genre, this was a very big leap.
2. The story - it was great, and even the parts that were horrible, were great in a campy sense of it. The cinematic cutscenes were good, and the overall story is something like an over the top anime mixed with a Tom Clancy movie. Somehow it just blended perfectly. It found a way to do three things at once; Conclude a very complex series, work as an individual game and flow perfectly with the gameplay, both integrating into it and being paced well around it. It felt more grown up, like it was trying to be more than we were expecting out of it.
3. The audio and visual quality... audio wise, there is no chance any game comes close. There are other games with great audio, but not like this. The music, the voice acting, the sound quality, the 7.1 surround sound, in every area is unrivaled.
Now personally, I think the video quality is clearly top dog, and until I see Gears 2 on my TV, I'm not going to guarantee it'll look better. It may, but I don't know til I see it for myself. Forgetting all that, consider the video quality and the fact it's actually a pretty damn long game, spanning a lot of terrain and in a lot of different looking areas. I feel like that seperates it some from games like Gears 1, Uncharted, Assassin's Creed, ext, that have a somewhat persistent pallette and reuse a lot of textures.
4. The scope. It was just an epic game. I mean, I could go on and just list things that happened that were amazing but since you played it, you know...