Lol, looking at some of the old trailers to see what "hype" the they brought to the game before the game came out and i really started to notice all the placeholders that didn't make it into the game.

MW3 Multiplayer Reveal Trailer

Notes:
In Underground the Trailer has Delta Force fighting the Inner Circle, In game its the SAS vs the Spetnaz. In fact it seems that only the Delta force and Inner Circle models were finished because none of the maps that were used have the Delta Force fighting the Inner Circle.

At 1:47 the Flag uses the insignia of the Navy Seals from MW2, Same as the Airdrop Trap.

A lot of the weapons used different firing sounds (Probably for dramatic effect) such as the M4A1 using the MK14 firing sound during the CTF segment. Also the Sniper rifle the enemy sniper uses always uses a M40A5 firing sound (from COD4) despite the fact that he is using the MSR.

MW3 Tango Down Trailer

Notes:

So many Killstreak Placeholders!, Escort Airdrop, Ballistic Vests, Advanced UAV are the obvious ones but if you look closer it seems that none of the Killstreak icons were finalized at that point.
Blast Shield Hitmaker at 0:54
MW2 Scavenger emblem at 1:04
Old Spetnaz emblem at 1:06?

Reaper at the end of the trailer uses the Javelin firing sound

MW3 Spec Ops Reveal Trailer

Notes:

Alot of MW2 firing sounds, the ACR 6.8 uses the MW2 ACR firing sound, same with the MP5 (using the MP5k firing sound)
MP5 model in the trailer has a grip on it which is not seen in multiplayer. (However it is seen it the single player level Stronghold) and is most easily seen at 0:48
MW2 Red Dot Sight is used a lot in the trailer (which is odd given as the MW3 variant is there as well)
G36C uses its old COD4 reload animation at :40
It would seem that a lot more perks were possibly going to make it into survival at ) 0:31, additionally Extreme Conditioning was using its COD4 tier and look (Extreme Conditioning is a tier 3 perk in COD4 but a tier 1 perk in MW3)
At 1:25, the AK-47 is using the HAMR scope despite the fact that in-game it can only use the Hybrid sight. In fact neither the Hybrid sight or the HAMR scope can be used in Survival Mode

Any other oddities you can spot in the other trailers?