I only play multiplayer and use a PC. All other modes are for people doing it wrong
The game itself is clearly built entirely on MW2. The files, configs and general look are pretty much the same. If you looked at a folder on a PC containing MW2 and one containing MW3 they're almost indentical. They've changed the menus around, beefed up the sound (guns sound a lot more powerful and loud) and I think they've made the graphics ever so slightly better but for all intents and purposes it's the same sort of stuff.
The developers have clearly focussed their efforts on the new maps, game modes, weapons and pointstreaks. In short they've spent their time on stuff you use rather than stuff you look at. The results are pretty spectacular. I've spent quite a few hours playing it online now and I've been laughing my ass off at times at how good it is.
Kill Confirm mode is just mega fun. Basically you can't sit still for very long or you'll get shot in the back. It's fast and frantic. If you do manage to find a little spot to camp in, you're likely to get some kills from guys running around clueless. Do you rush for the tags or sit and wait? It's a dilemma sometimes. But it's a simple tactical depth that the old game lacked.
Team Defender is more of the same but in a slightly different way. Just fast and frantic but more focussed. Only played it a few times but the flag tends to end up in one part of the map and both sides just swarm after it.
Having dedicated servers on the PC and making them unranked, frankly, is a master stroke. Whoever thought it up needs a medal. Since the news came out that it was unranked the abuse has been relentless. People have said they'll boycott the game over it but once you play it makes perfect sense. If you like ranking up like before do it through matchmaking. It's not perfect but the player numbers are high so you usually get a good game and decent enough ping. But for dedicated servers you can create 5 of your own classes, straight away, using any weapon or unlock. No waiting around to Level 80 to get the RPG - just enable it and jump into a server and use it. I think if they'd made server play ranked it would end up like MW1, which is a great game, but full of "pros" who have everything and destroy more casual players with ease. In MW3 allowing everyone access to every item/weapon works a treat.
At the end of the day it gives people a choice too. You can choose to unlock stuff but when you get tired out it's nice to just pick a game mode, map and server and go and blast around using any pointstreak, perk or item.
The pointstreak system is really well thought out. Having 3 different classes of pointstreak each with benefits and advantages really helps. If you're a bit of a camper or just really good at aiming you can still benefit from the older style "assault" streaks with helicopters and AC130's etc. If you're more of a rush type player who tries to complete more objectives but therefore dies a bit more trying to do them, the support streaks, which don't reset when you die, are great.
The weapons seem varied and some do appear to be overpowered. L118 sniper appears to not even need to be aimed (been killed quite a few times by people using it who haven't even scoped) and the UMP is a beast. I understand this happens in all games and usually after some extended play and feedback, patches help balance it a bit more.
The maps all appear to be a bit more complex than in MW2. They seem a little more "maze-y" with more paths. Quite a number are a lot more vertical with underground sections, buildings, windows and staircases galore which mean you do end up getting attacked from all sorts of directions. I suspect that, in time, I'll learn the maps better and get a feel for them which will make them feel "simpler".
The new items like bouncing bettys, turrets, remote drones and stuff like a portable scrambler or portable radar really add extra elements and tactics to the game. In MW2 you were more limited to grenades and some explosives but there are definitely more options now.
Hit detection feels better and when playing on a server it's definitely sharper. Literally "BOOM YOU'RE DEAD".
Overall the changes made make this game everything MW2 should have been. Most of it's flaws and failings seem to have been intelligently addressed and their are plenty of fun new modes (Kill Confirm and GunGame for example) which are great laughs. The game is definitely more tactical and less focussed on camping to get your killstreak built up.
I do wish they'd made the graphics a touch better. It's not that it looks bad or anything. It just doesn't feel like much time was spent, or available, on doing much beyond a few new models and small graphical elements. The plus is that it should run as smooth as silk on anyone with a fairly decent computer.