I think the deal with CoD4 is that it seems to give people a sense of realistic game play while being part of a popular series. Personally, I 'WAS' kinda impressed at the accuracy in some of the weapons, such as the cross-hairs in some of the sights looking almost exactly as they do in real life. Another thing is the greater reliance on the sights as opposed to the previous games, rather than on-screen reticle (again, trying to be more true to life), something I've always found adds a nice touch to games.
HOWEVER, when it boils right down to it, it still CoD, and plays a lot like the others. Sure, it's fun, and it deserves praise, but not as much as it's getting. Single-player seemed disappointingly short while multi-player goes a bit overboard with the 'upgrades' you can acquire like "Deep Impact" (Yeah, that bullet just went through a foot of concrete), which to me seemed like taking a huge step back from the realism the game was trying for.
Overall, good game, fun, but definitely not worth the hype. If you want a great game with true realistic feel and play, take a look at Insurgency (Mod for HL2) or Project Reality (Mod for BF2). Both are MUCH better than CoD 4 (In My Honest Opinion) in terms of online play and entertainment value.