The people giving this poor reviews are obviously haters, BF3 boosters, or both.
Look, I've played these since the original run and as far as the single player goes, the mechanics haven't changed much (hit the 'trigger' opponent, move to next fight scene, ad nauseum; the AI could be improved also with it still reacting stupidly between friend and foe when they encounter one another) which is why I'm giving it 8/10. The single player story however, looks great and is still interesting enough to make you want to play it. But who's buying these simply for the campaign alone anyway?
The multiplayer is the best out there, period. The framerate makes the game unmatched, yes even those who have supposedly introduced bigger and better engines (played 'the other guy' and didn't see what was so fabulous), along with the great upgrade system whereby you can truly make your playstyle your own. There is no 'right' or 'wrong' setup, but the one which works for you. The devs had said prior to release they're aiming at making it a gunbattle multiplayer moreso this time around and took out the complete dominance of airpower, and they did. The options are near limitless, and a lot of fun to explore.
Then, the brilliant idea to keep the hook in: Elite. The fact that they're going to release one map per month is very smart, as what usually kills a game off is the lack of maps; I know it certainly is for me. Definite thumbs up there.
So that is two games in one, but we're still forgetting a third: Spec Ops. These are a lot of fun also and they've improved far beyond what MW2 offered, as you now level up separately in Spec Ops with the bonuses from the missions contributing to the Survival mode. Very cool idea. Again, they've beaten the pants off it's braggart competitor, BF3 in this regard. I played and beat all of the missions in BF3 with little effort in a few hours. To make matters worse, if you want to unlock all the in game bonuses, you're forced to play through the BF spec ops missions again and again and again...ugh. The winner of that battle is clear cut, I believe. But this review isn't one game vs another (as they're two differing games that shouldn't be compared to begin with honestly, but I've drawn myself into the silly debate by offering my comments I suppose), it's about THIS game.
To summarize: the single player mechanics are a letdown, as it's the same formula but the story is still fun to play through, which I recommend doing, especially if you've followed the series since the original MW. The multiplayer has ratcheted up the fun factor with a vast array of options, and the Spec Ops is well done, with both missions and survival mode. Giving this game a bad review simply isn't honest. Proves the old adage though, that when you're king of the hill everyone either loves you or hates you. Well done, IW/Activision, I look forward to the next one. :)