Thanks to the user CoD4, I decided to check out both the game's files and modding tools to see what stuff is unused in Call of Duty 4. I'm not sure where most of this would fit, so I'm dumping it here.
Alt Grenade Launchers
At one time, the G36 and G3 were supposed to have their correct grenade launchers. There's also a listing for the SCAR-L/H's grenade launcher in there as well.
At one time, the Claymore would behave much like the C4, where you'd have to plant it and detonate it manually.
Selectable Fire Modes
An overview for an unused training map has the text "M4 Carbine Full-A" on it, with the A assumingly being "Auto". If so, that means that you could select a gun's fire mode, like in CoD1.
A text string is called "PRESTIGE_CAMO". I'm guessing you were supposed to be able to get a special camo after prestiging at one time.
Unused MP maps
There are a TON of overviews for unused MP maps.
It seems that Broadcast was supposed to be originally included in the game, as a rough overview of it, similar to the overviews for the unused MP maps, is in the main files.
The TV station in Charlie Don't Surf
Apparently, the TV station is (or was) supposed to be run by Al-Jazeera, as a texture for it is called "armada(CDS's internal name)_ajazeera_worldmap".
A background for an alternate version of Crash can be found in the files. Apparently, the crashed helicopter was supposed to be a Black Hawk at one time, the map was much dustier, like in Charlie Don't Surf and the area around the chopper was much smaller.
In CoD2, whenever you completed a country's campaign, you would get a "VICTORY!" splash screen at the end of the map with images of happy Russians/Brits/Americans, depending on your faction. In CoD4, there are such images for the US Marines and the SAS. This wouldn't make sense unless it was possible to play their levels as seperate campaigns instead of one big one, like in the final game. Maybe, at one time, the Marines and SAS had different campaigns that weren't interwoven?
Different Create-a-Class images
It seems that at one time, whenever you attached something to a gun, the gun's Create-a-Class image would reflect that. Like, if you attached a Gl to the M4, the CaC image would be an M4 with Gl. What's interesting to note is that the unused modifed CaC images are slightly different compared to the final's, mostly in lighting and size. The only one that is significantly differnt is the silenced Skorpion's, which has a black body and an orange grip. It seems that this feature was scrapped way before the final CaC images were made.
Different kill images
Speaking of attachments, it seems that the image that would appear whenever you killed someone would reflect the gun's attachments. IE, if you have an M16 with the GL attached, the icon that appears when you kill someone with it would have a GL on it as well. This is a neat detail, and I don't know why anyone would scrap it.
At one time, there was supposed to be a green reskin for the P90.