Meaningful Release Date: November 10th, 2009 for Modern Warfare 2, which was advertised in the trailers as "11-10-09", with the numbers blinking on in sequence like the countdown before an explosion.
Name's the Same: One of the randomly-generated names for the FSO agents in the MW3 level "Turbulence" is, minus a single I, the exact same as that of World at War's Russian Player Character.
There's also a Pvt. Allen in "The Bog" in Modern Warfare 1.
What Could Have Been: In spring of 2011, a lot of info about Modern Warfare 3 was leaked. including concept art and the game's plot (You can read it here). While most of the plot is similar to what made it into the final product, there are several interesting differences such as a mission set in the Kremlin and Sandman participating in the final mission to kill Makarov.