Oghren turned into a bumbling drunken parody of himself ; any character development he had by the end of DAO was retconned away just like those epilogue slides , Anders became a terrorist and now it's Varric's turn.
You can see why I'm not a fan of returning characters.
Gaider was forced to include Oghren in Awakening and if you pay attention to his characterization, it's not a retcon but a deliberate backslide. He's a messed up individual in Origins and despite his accomplishments with the Warden, he never really addressed his problems (being rejected by Branka and losing his place as a warrior, her actions/death, his alcoholism). His arc in Awakening is about making him confront his unresolved Branka issues and deal with his responsibilities a little more head on.
Anders, too, was not a complete asspull...his actions in DA2 were foreshadowed quite a bit in Awakening.
Varric does not have Oghren's demons, nor does he have the amount of anger in him that Anders had even when he was "happy". Nothing we've seen or heard about him has indicated that he's made a massive u-turn, personalitywise, or even a minor swerve.
As for Cullen...eh, his backstory already covers some extreme swings in behavior, and we know so little about who he is outside of being a templar with PTSD that I don't know why people are freaking out about him possibly being in DAI and being ruined and/or ruining the game.