Executive Meddling: Microsoft pushing the release date of Gears of War 3 back five whole months from April 2011 to September 2011.
Arguably a positive example of meddling from higherups: Epic Games may not have done a multiplayer beta if this had been released as scheduled and they themselves seemed happy about a chance to polish everything to a spit shine.
Game Engine: This is the first Unreal Engine 3 game to be released, oddly enough NOT being an Unreal title or a PC game (initially) like the debut games for earlier engine versions. Early Unreal Engine 3 tech demos from 2004 even used Gears Of War assets rather than Unreal ones. Gears of War was made largely as a demo for the engine, with each succeeding game further enhancing it's capabilities. It's worked well, as several very popular games have been made with the engine.
Mythology Gag: Clayton Carmine◊ is a big one to his decreased younger brother Anthony. In Gears of War, Anthony was killed by sniper fire after standing up to display his issue with his Lancer assault rifle jamming constantly as he reloaded it. To list how Clayton's design references it, "Practice reloading" and "Keep your head down" are written on his armor. If that wasn't enough, a mark on his helmet implies he was shot in the head but survived anyway.
Clayton honors the memory of the two fallen Carmine brothers with a memorial tattoo on his arm of their helmets.
Gears of War 3 is filled with these.
The tier 4 medal for interacting with objects during the campaign (i.e. hitting buttons) references Lieutenant Minh Young Kim's oft-repeated line in the first game of "I'll open the door; I've got the code."
A player using Dominic Santiago will sometimes hear him repeat one of his earliest lines from Gears of War, "This is gonna be awesome!"
After killing a COG character, Queen Myrrah will sometimes reference a line from the Gears of War 2 opening cinematic, "You brought this upon yourselves!"
Early in the first game, after first meeting Augustus Cole, Dom and Marcus talk about when they saw him play. Marcus will quip "I remember you owe me 20 bucks" to which Dom replies "Yeah? See me after the war." One of the first collectibles in the campaign is a 20-dollar bill on Marcus' desk with a note from Dom.
If you can manage to get through a round of Gridlock without anyone destroying one of the ash people, the next round will begin with an instrumental version of Gary Jules' cover of "Mad World", the same song used for the Gears of War commercial. It's also the music that plays during Dom's Heroic Sacrifice.
"Who wants toast?!" makes a comeback in Gears of War 3 as one of Marcus' one-liners when throwing an incendiary grenade.
Gears of War 2 has no shortage of them either, even though it only has one game to Call Back to. "This is the part where he tells you he'll take it under advisement." and "Hey, Colonel, I guess we ARE the support, huh?!" come to mind. Also, the entire time you're with Benjamin Carmine in the Hollow, Dom doesn't stop making stealthy quips about Anthony, even crossing over into Dude, Not Funny! territory.
What Could Have Been: The series staple, the Lancer/Chainsaw Bayonet, was on the edge of whether it was going to be actually included in the game. It wasn't until they devised the design of it being integrated into the grip that they devised a semi-plausible chainsaw mounted assault rifle.