All weapons have a maximum and minimum damage value. Weapons do their max damage up until a certain distance when the damage begins to decrease and eventually levels out at the min damage value. Silencers will cause this falloff to occur earlier, and thus do less damage at longer ranges.
Some weapons don't experience any falloff, and maintain their max damage values at any range (LMG, Snipers). Equipping a silencer on these weapons will permanently drop the weapons damage to it's minimum damage, regardless of range, so you'll get the minimum damage no matter how close or far away you are.
The only weapon that violates both of these rules is the UMP45, which always does it's max damage, regardless of distance or attachments. A silenced UMP45 at long range is just as powerful as an unsilenced UMP45 at close range.
Different areas of the body still have their own damage multipliers, though there seems to be a slightly simplified model (no "neck" hit area, as this is now grouped together with the upper body / chest area). The damage multiplier for the head is still 1.4 or 1.5 like it was in COD4. Sniper Rifles still remain the only weapons to take full advantage of the body part multipliers, and use the same damage x multiplier system that was in COD4.
Stopping Power still multiplies weapon damage by 1.4 on top of any other multipliers.



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Equipment
Frag - Can throw a little further than Semtex, and can be primed, but does approx. half the damage (thus half the killing range) of the Semtex. Also, displays location icon and can be thrown back if nearby. Can't be canceled by switching weapons prior to throwing.
Semtex - Doesn't throw as far as the Frag, but it sticks to whatever it touches, doesn't roll, does approx. twice the damage (thus twice the killing range), doesn't display location icon, and can't be thrown back. The Semtex can't be primed, but it explodes on short timer that starts the moment it leaves your hand. Can be canceled by switching weapons prior to throwing.
Throwing Knife - Travels for several meters in a straight line before beginning to drop. Kills with a single hit anywhere on the body, and can hit after bouncing off other objects. Can be canceled by switching weapons prior to throwing.
Tactical Insertion - Takes three seconds to lay down after pressing. Can be destroyed at a distance by shooting, etc.
Blast Shield - Reduces explosive damage by approx. half. Will allow players to survive point-blank explosion of Frag or Grenade Launcher. Semtex, Claymore and C4 will still kill within a meter or so. Upon activating, will narrow your vision and remove the mini-map. Deactivation will restore the mini-map and normal vision. Activation / deactivation is fairly quick and can be done on the fly.
Claymore - Explodes 1-1.5 seconds after tripping. Does approx. same damage as Semtex and C4. Can only set two at any time, if you set a third, the first one will explode.
C4 - Does approx same damage as Semtex and Claymore. Can only set two at any time, if you set a third, the first one will explode. Can also be detonated by pressing the reload button twice rapidly.


Launchers
The AT4-HS with Danger Close Pro is still unable to take down any air support vehicles in a single shot, other than the UAV/Counter-UAV.
The Javelin can lock onto distant terrain and fire a missile that does explosive damage to a very large area, killing anything within line-of-sight. As powerful as this sounds, it can be challenging to use this capability effectively.


Miscellaneous
Iron sights have little, if any, effect on shotgun spread / accuracy.
Extended Mags do not provide any additional ammunition, it merely extends the round capacity of the weapon in question by half (confirm?).


Differences Between COD4 and MW2
Base weapon damage values in MW2 are on average very similar to COD4, with no substantial increase in MW2 over COD4.
Weapons have much longer range in MW2, retaining their max damage for a much greater distance.
Weapon "idle" (reticule sway) has been greatly reduced in MW2 and is virtually non-existent except for on scoped weapons such as Sniper, ACOG, or Thermal.
Weapon "kick" (recoil) has been greatly reduced in MW2, particularly for assault rifles, and is the chief factor in affecting how much more deadly the weapons in MW2 are compared to COD4.
Hip fire is much more accurate in MW2, and sub machine guns no long hold a significant advantage over other weapons in this area.
As far as I can tell, hit detection and lag compensation in MW2 is improved over COD4, reducing the lag advantage for faster moving targets and allowing more rounds to make contact as intended instead of missing due to lag / poor netcode.
All of the above factors make the weapons in MW2 much more deadly than in COD4.