Executive Meddling: One of the factors that allegedly led to ACM turning out the way it did was Sega mandating that the game be more like Call of Duty, with the Marines focusing on fighting other humans rather than Xenomorphs. This was reflected in a negative reaction of a lot of reviewers, who were expecting more Xenomorph-fighting.
Fandom Nod: As noted on an XBox Magazine pre-release feature about the game, Bishop's first line is the ad-libbed "Don't ask me to do the knife trick. I can see it in your eyes." This references actor Lance Henriksen's constant requests to do said trick by many of the fans he met over the years.
Saved from Development Hell: Colonial Marines is infamous for its numerous delays and production problems. An early version (developed by Check Six Games and published by Electronic Arts), featuring the same concept - Marines go looking for the Sulaco after the events of Aliens - was announced in 2001 and cancelled sometime later. Sega announced they were working on a new Colonial Marines game (apparently unrelated to the Check Six version) in 2006, and the game was subsequently teased and then delayed multiple times. It finally arrived on February 12, 2013. Even then, several reviewers said it needed a lot more time on the polishing wheel.
Troubled Production: Very troubled. Fox had been trying to license the franchise to a qualified developer for over a decade, beginning with an aborted attempt by Check Six Games in 2001, then lining up Gearbox for the task in 2006. However, production was slow, there were a lot of "cooks in the kitchen" so to speak with different ideas about where to take the franchise, and allegedly, Gearbox was using money Sega paid them to work on other projects, rather than Aliens. In 2010 Gearbox subcontracted development of the single player campaign to Timegate so they could have time to work on Borderlands 2, who allegedly discarded much of the work done to that point and began again from scratch. By 2012, most of their replacement assets were still incomplete, and Sega was becoming impatient with progress to the point that they could threaten legal action for contract breaching. Gearbox had to step up, try again from an incomplete product, and rush the game out the door despite knowing it was in no condition to hit the market in order to get their contract fulfilled. With the reception of Duke Nukem Forever still fresh in their memory, this must have stung...
From Timegate's side, insiders have alleged that Gearbox had sent them very few assets to work with, certainly not four years' worth of material, and what they did have didn't work, which was why they had to scrap most of it. Also, the script still hadn't been finalized by that point, so Timegate was forced to continually scrap or redo content due to last minute story changes. There was also the fact that Timegate and Gearbox had diametrically opposed design philosophies. Gearbox usually takes its time with its titles to maximize quality, while Timegate has traditionally worked to get games shipped as fast as possible. The end result of the mess was a game that was hammered out in nine months from scratch that didn't satisfy anybody.
What Could Have Been: A large amount of content was either stripped from the game or changed in pre-production, including:
The E3 2011 demo, which does not appear in the same shape or form during the final product. The demo included different characters (a CO who had a robotic arm, Keyes - who was present on Hadley's Hope and not just in the Sulaco), different setpieces (defending the access tunnel from a horde of xenomorphs with other Marines and multiple sentry guns, getting knocked out of Operations by a Xeno, having to outrun a Charger, fighting a Queen in the Hadley's Hope maintenance bay), and many different gameplay elements, including dynamic lighting, different backgrounds on Hadley's Hope and enemy corpses that didn't disappear.
Several moments seen in pre-release trailers (including a shot of the Sulaco and Sephora crashing to the surface of Hadley's Hope) never appear in the game itself.
The first single-player gameplay demo (from 2012) had a wide array of changes during the opening "Distress" mission as opposed to the final product. There's blood left throughout the Sulaco umbilical bridge, O'Neal is introduced when the door to the hangar bay is opened (not tending to a downed Marine), Winter has more dialogue with his CO in the Sulaco hive, and the player had the ability to wield a smartgun during the xenomorph assault in said hangar bay, among other things.
The game was originally planned to have no HUD.
A Wii U version of Colonial Marines was supposed to be released by the end of March under Demiurge Studio, but due to the negative reception, SEGA canceled it.