In the "Gat Is Back" Trailer, Gat says "You thinking I wanted to go to Hawaii or something?", a reference to Daniel Dae Kim's role in Hawaii Five-0, and to the rumor that Gat "died" in the previous game because Kim was too occupied with the show to provide more lines, which he later commented as wrong.
The line Female Voice 1 says while doing the "Quarter Circle Forward + A" taunt is "Spinning Bird Punch!". Laura Bailey also voices Street Fighter's Chun-Li, who possesses a Spinning Bird attack—albeit, one that's a kick.
If you pick the Nolan North voice, then you'll get plenty of these, since Nolan North is apparently playing himself.
Keith David has a few of these.
During one of the cutscenes, Kinzie talks with him about how he looks and sounds a lot like Julius Little, the original leader of the Saints. Keith quickly denies this. This becomes a Running Gag throughout the game.
Immediately prior to said mission, there's another one where supporters of Keith David show up to a rally with "I <3 Goliath" signs.
In the previous game, it was stated that Michael Clarke Duncan (who originally voiced Benjamin King) would play the lead role in the movie based on King's autobiography. In this game, Roddy Piper tells King (now voiced by Terry Crews after Duncan's death) that he liked the movie, but "the guy who played you sounds nothing like you."
Bad Export for You: The Australian version, due to the removal of Shaundi's loyalty mission.
Banned In Australia: Even with the R18+ rating now being available for video games, they still banned the game for containing "interactive, visual depictions of implied sexual violence which are not justified by context" and "elements of illicit or proscribed drug use related to incentives or rewards." All Deep Silver had to do was remove the one 20 minute mission that had drugs and the game has been accepted, at an MA rating. Unfortunately, the exclusion of that mission means no one in Australia can play co-op with anyone except other Australians. Volition responded to this discrepancy by making the American/European versions of the game region-free for the Xbox 360.
Lin, Donnie, William Sharp, Dane Vogel, Viola, Loren, and Zimos were originally going to be homies in game and had cut audio logs. Zimos' clothes are available for purchase in stores, and his personal symbol shows up on the Saints' mothership, so it is possible they may return in DLC episodes. Zimos has indeed returned in Enter the Dominatrix.
The "aliens trap the Saints in a virtual Steelport" aspect was originally going to be a stand-alone expansion for The Third, called Enter the Dominatrix. It was later decided to cancel the expansion and just merge that and the concept for IV together. Who knows what could have happened if the "virtual Steelport" and "the Boss as the President" plots stayed separate.
Later on, Enter The Dominatrix was made a DLC pack, and several previous characters do indeed return, answering both of the above points.