Echoing what everyone else already said, plus adding a few of my own. I played mainly red- and goldside when the game was live and only really explored blueside in the last year or so.
Redside mission goal descriptions are more straightforward. For example, it will be explicit if it is a defeat all or not. Blueside mission goal descriptions can be ambiguous sometimes, such as it might be a defeat all without saying so
Speaking of mission goals, redside rescue missions involve bringing the captive back to the mission exit whereas older blueside rescue missions do not and just require you to defeat their captors
Much to my disappointment as a temporary power collector, some blueside missions that give you a temp power either remove it at the end of the mission or you have a time limit for how long you can keep it (different than how long you can have it active)
In general, redside missions are more solo-friendly
There seems to be a lot more hunt missions on older blueside content
While redside and newer content have contacts and story arcs with more personality, I like that older blueside contacts are often just normal people (and some magicians, mystics, etc.) who work with heroes. It handles how are hazardous waste disposal, civil rights, corporate espionage, doing journalism, etc. handled in a setting with heroes, villains, and superpowers