Each of the heists is based on a movie. For example, the "Heat Street" heist, involving an open-air shootout with the cops on crowded city streets, is heavily inspired by Heat.
When the crew's would-be getaway driver is shot and crashes his car, it's also reminiscent of when Donald Breedman, the getaway driver for Neil McCauley, gets shot in the head while escaping the police. The getaway cars are even nearly identical to each other.
Have an AI teammate in the bank lobby at the start of the "First World Bank" heist. Neil McCauley's line from the bank robbery scene gets quoted, word for word.
The music while casing the First World Bank is very similar to the music used during the bank robbery scene in Heat leading up to the famous shoot-out.
There is one to Left 4 Dead in the form of an achievement, Left for Dead, which you get for finishing a heist in custody while all three of your teammates escape, something that is very common in finales of... wait for it... Left 4 Dead.
The 7th Patch added zombie-related movie-posters to "Panic Room" and some very familiar red first-aid-kits - four in a room only available in Overkill 145+, two in the cellar you escape through. In addition, "Diamond Heist" got a few posters with a young woman, which looks a lot like Zoey with a bit longer hair and no ponytail. Then there's the whole No Mercy campaign, which takes place before the events of Left 4 Dead at Mercy Hospital. It's been stated it's not a prequel, however.
Also in the No Mercy level, if your escape route takes you through the basement, Bain declares you are Left for Dead.
Also also, sometimes Bain's pre-heist briefing is interrupted by a certain bearded Vietnam vet getting on the elevator and striking up a conversation with Wolf ("Insurance salesmen, bunch of crooks if you ask me").
When you answer the phone, Bain will sometime tell the person calling that there was a "weapons malfunction" just like Han from Star Wars.
Another achievement, for finishing a heist having killed a bulldozer and not having taken damage from any, is called Dodge This!
Wilson and Michell's houses are protected by White Mesa Home Security Systems.
If Alex successfully lands the limo on the roof on Undercover, he will say ''Like a glove!''
The "Cheney?" achievement is earned by killing a Bulldozer with a shotgun blast to his face, which is a nod to the infamous event where former vice president Dick Cheney wound up shooting his friend in the face by accident.
The alarm boxes in Diamond Heist sometimes have "guru meditation" errors according to Bain. "Guru meditation" was the Commodore Amiga equivalent of a BSOD. The player may also hear this message in Payday 2 while hacking a computer.
Various phones seen in heists show 'idKFa12321' on the small LCD screen, in reference to the user of the same name who was among the "Lucky 200" in Payday: The Heist. They also appear in some heists in the sequel as well as the safe house.
The crystal meth you cook in the first day of "Rats" is blue. If that was too subtle, Bain may say you should "break bad" and go into full-time meth-cooking if you you cook extra meth (the additional bags bringing a significant amount of extra cash in on an already highly profitable mission) or do well in general.
Generic GenSec Personnel on Armoured Transport heists wear red uniforms.
Generic security guards are also wearing red undershirts under their white uniforms.
When a standard enemy is killed by a trip mine deployed by Chains, he will sometimes quip "Surprise, muthafucka!" (This line also comes up when he is tossing grenades, regardless of if they actually hit anything)