FIX: main/iw_12.iwd is different from the server
This works for a short period of time apparently, I am going to be updating this post with the offical post from steam and the files they gave us to update our steam folders. It seems they are pressing us (non DLC holders) to buy the DLCs. Steam released the 2 files from the DLCs so we will not get this error anymore. These files are from the DLC package but we will not be able to play the new maps without purchasing the DLC package.
So here it is, the official temporary fix for (most) DLC issues. Do note that this only works for people who don't have the DLC. If you do, stop reading.
Download iw_25.iwd and iw_26.iwd
Move the files you just downloaded to "C:\Program Files\Steam\steamapps\common\call of duty modern warfare 3\main" (note: this is "Program Files (x86)" on 64-bit systems)
Enjoy the game again!
Important note:
Downloading these files will not allow you to play the DLC without purchasing.
This quote is from steampowered forums. It can be found here:Official temporary fix for non-DLC owners - Steam Users' Forums
I was going to post the links to the downloads but they are too large for a virus scan.