I'm just going to agree with Dreddz, in an attempt not to accept Manus' invitation to dissect everything he's saying point by point.

The stealth was incredible. The fact that you didn't have to play that way, but hiding in plain sight, multiple paths, and having a million options to handle every task made it, easily, the most rewarding stealth experience in the series. Not only that, but the variety of situations was incredible. Act II especially where you infiltrate an execution site, and then can choose infiltrate with a group of rebels, following their lead. Finally is Prague, where you essentially help someone else infiltrate. The game play went in bold directions that couldn't even be predicted by previous entries in the series.

Finally, the multiple paths and and forks makes the game, at least in one respect, much better than the previous entries. I've beaten the game 3 times and started it a few more, and I'm still finding new things and ways to infiltrate. MGS4 easily has the most replayability in the series.

I think some of you guys are placing too much on the "infiltration of the battlefield" aspect, which is only in the first two acts, and it hardly encompasses the entirety of those missions. Those are really the only open invitations to run and gun, I simply couldn't imagine playing the rest of the game without stealth.