There are plenty to mention:
Five new classes
Some of the classes have going through change (Illuminati chief does give Ops Bonus but to non-illuminati players is because the fake feedback gives you false sense that instead does not replenish uses at all, Vigilante/Trooper will not be penalized by killing Illuminati through green colors, Leader does not frame but also make their NSF members not be seen as NSF/Illuminati but UNATCO instead)
Illuminati have no chat except in NSF
Illuminati only know each other by name so no class mentioned
The ātriggerā of revealing Illuminati is by either killing all the members or kill Rogue Agent early in order to flip blue instead of green or red in the graveyard and further flips.