So like, is there any difference what so ever between this and CoD4?
Here's a list I've been compiling from around the web. Some of it might need some updating:
New Maps: Obviously, right?
In addition, the engine has been upgraded with new texture streaming that allows for bigger maps with more detail. So far, 13 multiplayer maps have been confirmed.
New Guns: In addition to the old guns, some new stuff to play with such as the UMP 45, SCAR-H, FAMAS, AUG A2, SA80, F2000, Bushmaster ACR, HK-416, MTAR 21, AUG HBAR, FN FAL, etc. You can also now unlock Rocket Launchers to use as your secondary weapon, even one that will lockon to helicopters.
Complete list of confirmed weapons with pics:
http://www.infinityward.com/forum/viewtopic.php?f=13&t=115738&start=0
New Attachments: For instance: you can now unlock a Heartbeat Sensor that is a little LCD screen that sits on the top of your gun and detects heartbeats in the area around you so it's a cool way to find hidden enemies.
New Game Modes: First off, a new version of Search and Destroy is now supported in the game. Titled "Demolition," this new mode (whether it replaces S&D or is just in addition to it) allows for everyone on the attacking team to have a bomb, and full-on respawns as the round goes on. Defenders try to stop the attacks at both an A and B site, but if attackers succeed in a single bombing their time is increased and the focus shifts to the other site entirely. Also, Capture the Flag is back in a mode called "Flag Runner".
Customizable Kill Streaks.: You now unlock kill streaks as you rank up. Some of the kill streak rewards require three, four, five, six, seven, nine kills?
New Kill Streak Rewards: There are 15 total killstreak rewards for you to unlock. In addition to old favorites like the UAV, there are things like a care package that?s parachuted in and grants you a random upgrade; stationary sentry guns you can put down to beef up defenses, especially in objective-based games; the new AC130 gunship (you can actually control the gunner of it), which requires 11 kills to use. Other custom killstreaks are predator missiles and Anit-UAV (basically a UAV Jammer for your whole team). The predator missile is a cam-controlled drop where you can actually steer the missile with a laptop computer. Supply drops are interesting as well, having players throw down smoke that actually brings in a helicopter drop. Anyone on the field can use it though, so it's a race to get to the box. It can contain any of your killstreak options currently set (basically a roulette) and can seriously turn the tide of a battle.
There is also a new version of the Airstrike, called the Precision Airstrike, which now lets you change the approach direction of the attack.
Also, in addition to the regular helicopter, there is also a new helicopter with heavier armor.
Death Streaks: Death streaks? Yes, you will now be rewarded for dying too many times in a row, in an effort to prevent frustration for those just getting into Modern Warfare 2. For instance, there's Painkiller, which starts you with a triple health boost after you?re killed three times without making any kills of your own, and Copycat, which lets you use the loadout and perks of the last guy who killed you for one life.
Secondary Weapons Replace Pistols: You no longer have to waste a perk on Overkill, now everyone can take two primary weapons, or the pistols are still an option if you'd like one.
Dual Wield Pistols: One for each hand.
New "Bling" perk Allows you to have two attachments at a time! Now you can attach both a silencer and a red dot, or ACOG, or whatever.
Lightweight Perk A perk to make you walk faster.
Marathon Perk A perk to give you infinite sprinting
Commando Perk: A perk to increase your knifing range
Scavenger Perk: allows you to re-up on all supplies and ammo any time you cross over a dead body.
Cold-Blooded Perk: Makes you undetectable by UAV, Air Support, sentries, and thermal sensors.
Health Boost Perk: This is for those people who hate being spawn killed: a perk that gives you automatic health boosts for 10 seconds after respawning.
Hardline Perk lets you get to your customized killstreaks one kill faster (if UAV is at three, you get it instead at two kills every time)
Pro Perks. Not sure exactly how I feel about one, but we'll see how balanced it is: Pro Perks are new additions to the game, allowing for you to actually increase the power of a specific perk (say, stopping power or increased speed) if you use it a lot. Kill a set amount of people with stopping power on and you unlock "Stopping Power Pro." These perks can be seen on the main perk selection menu, and like all the other customization in the game it's entirely streamlined. If you hover over any attachment, gun, or perk in the game and click it, you'll see info on that selection, including how many kills you need for the next challenge or unlock.
Fancy New Equipment Instead of Grenades. Toss out your frag grenades, if you want, and replace them with some nifty gear that serves a variety of other purposes. Perhaps you would prefer Semtex timed sticky explosives? How about some throwing knives? The most abstract but perhaps coolest piece of equipment is the "Tactical Insertion", which looks like a typical flare when you throw it down. In practice, it acts as a mobile, single-use spawn point, so anytime you?re moving into a heated area, throw one down and that?s where you?ll respawn if you die. You can also sub your grenades out for a Blast Shield which protects you from explosions.
Experience Boosts Galore. It seems like you get experience point bonuses for doing almost anything in the new game. I?m not just talking headshots; any stylish or meaningful kill is going to give you some kind of experience boost. Kill a guy just before he?s about to get a kill streak? Bonus. Take out the guy who just killed you? Bonus. Get the last kill in a match? Bonus. IW says the leveling (which now caps at level 70) has been tweaked to take the faster accumulation of points into account, and you?ll get something gameplay-related for prestiging this time around.
Callsigns. Infinity Ward has been playing a lot of Street Fighter IV. The title and emblem system from that game is reproduced almost exactly here, with dozens upon dozens of descriptive phrases you can unlock and slap on your profile to give other players a sense of exactly how you?ve been putting it down online. The titles and emblems you can get will correspond to your behaviors, so if you spend a lot of time camping or you rack up a ton of melee kills, everyone will know it.