Well guys, finally forced myself off of multiplayer long enough to beat campaign. I dont want to ruin it however, so I think I'll review multiplayer only. Enjoy campaign on your own =D

First off, there are several ways to earn lots of bonus experience. Too many to list even, i'll try...

You get experience from getting a kill (obviously). If you die and you kill your killer you get revenge points (x1 kill + kill points) also, if you get a long range kill the same happens. If you kill an enemy that was shooting someone on your team you get x2 kill for "savior" points. If you kill an enemy that just killed your ally you get "payback" points (x2 also). getting points is loads easier on MW2, but there are far more levels and unlockables now. The max level now is 75, then prestige. There are also a multitude of killstreak perks. you choose 3 perks out of a long list. The list must be unlocked first, through level ups. here are a few killstreak perks.

UAV-gives you and all allies temporary UAV scan.
UAV JAM- Jams enemy or enemy team UAV status.
SUPPORT CRATE- This gives you a red canister. You throw the canister in an open area and a dropoff plane will drop a crate containing any random killstreak perk (helicopters, UAV, etc.) *note* crate perk kills do not add to your killstreak. Crates are a large role in MW2 online. You can grab enemy crates before they get to them, and vice-versa. Its a wildcard.
PAVE LOW- Armored helicopter than destroys the enemy team
Tactical Nuke- Kills everyone on the field and ends the game. Requires 25 killstreak. EPIC!

Perks can be leveled up to a "pro" form if you complete required tasks to level them up. This makes the perks stronger.
Here are a few perks:
Bling- Allows you to put 2 attachments on a weapon instead of 1. Bling Pro- allows you to dual weild weapons, 1-H.
Stopping Power- Make bullet damage much stronger
Sleight of hand- Increased reloud speed
Scavenge- Resupply from enemy corpses
(con't in part 2)