Suicide Squad certainly left room for improvement (even after James Gunn came along to do it better). So it isn't surprising that Thunderbolts* is a more entertaining spectacle. But if you ask me why I think it's better, it's all about the casting. Granted, almost all of this casting took place years ago, as the MCU gathered up actors like Infinity Stones to set in their almighty gauntlet. But the way these performers come together really shows how expertly they were assembled along the way -- the perfect combination of established names, stars on the rise, and capable lesser-knowns.
Foremost, Thunderbolts* is as enjoyable as it is because Florence Pugh is the star. The movie is the quip-tastic extravaganza that everyone has come to expect from a Marvel team-up movie... but it also tries to have more heart than the last several MCU films have mustered. Pugh excels at both of these things, getting laughs for her flippant handling of the one-liners as she powers her way through to the movie's more dramatic moments. She's at the center of pretty much all of those, making her damaged character feel like more than a cliche, and her efforts to find good in the villain feel noble. Plus, she does it all with an accent that, coming from anyone else, would seem too goofy to believe.
A solid second is Sebastian Stan. While this is far from his first Marvel rodeo, he isn't phoning it in. Within the narrow confines of his "strong, silent type" character, he's great at letting the mask slip just enough to show his character's feelings and motivations. (And, of course, he's great in the action sequences.)
Even though the rest of the cast is neither up at Pugh's level nor given as much to do, most of them have a history of being good in exactly this type of entertainment, and so they serve this movie well. Wyatt Russell developed his action hero chops in Monarch: Legacy of Monsters, David Harbour did it in Stranger Things, and Hannah John-Kamen did it in Killjoys. They're all perfect to plug into this blockbuster team-up movie.
Meanwhile, Julia Louis-Dreyfus seems to be having a great time as her ongoing MCU bureaucrat. There's a fun bit of meta commentary in using her this way; after playing a bumbling narcissist for years on Veep, the MCU is showing us what happens if someone actively malicious and actually smart gets near the levers of power.
The plot is fine enough. Thunderbolts* does a better job of injecting personal stakes into the global catastrophe than many Marvel movies have done. But at the same time, the amount of "required reading" to enjoy this movie is off the charts; as someone who hasn't watched any MCU movie for a second time in nearly a decade, I often feel like I'm trying to carry water in my cupped hands when I watch one. And of course, Thunderbolts* was never going to be the movie to break that trend. (We'll see if the upcoming Fantastic Four can be the first truly stand-alone Marvel movie in years.)