1) Peggy Carter liked Steve Rogers before the Super-Soldier serum.
(EDIT: Since people keep misinterpreting my point here, we get canon confirmation about this point in What If whereas it’s only heavily implied in the MCU)
2) T’Challa wants to change the world for the better. When he was raised as a king, he approached world issues through more diplomatic means. When he was raised as a ravager, he took on a more direct approach. Regardless, T’Challa is a good dude who wants to help people.
3) Hank Pym has an inner darkness that he struggles with. We saw that in the “Ant-Man” movie when he told Darren Cross that he saw too much of himself in him. Then, we saw his darkness in full display in “What If…?” when he took out the Avengers.
4) Doctor Strange is extremely obsessive, to the point of being borderline reckless. We saw that with his determination to fix his hands, as well as his attempts to reverse Christine Palmer’s death.
5) Vision is just as madly in love with Wanda as she is with him. Wanda trapped a whole town in her made-up world just so she can settle down with Vision, whereas Vision fed people to zombie Wanda because he couldn’t bring himself to put her down.
6) Erik Killmonger is straight up a cruel human being. “Black Panther” actually portrayed him in a more sympathetic light by showing how horrible his childhood was and having him die peacefully. But in “What If…?”, you see the full extent of his cruelty.
7) Thor and Loki are dumbasses who just want to have a good time. Thor’s always been a himbo, but as we see in “What If…?”, he would’ve been far worse if he didn’t have people guiding him. And as for Loki, we already knew he was a party animal based on “Thor Ragnarok” and his own show. “What If…?” essentially said that he’d be an even bigger party animal had he been raised properly.