When it comes to the two most prominent superhero teams in comic books, the obvious choices are the Avengers and the Justice League. Each one is comprised of the best heroes from their respective universes has to offer. Back in the day when Marvel and DC were a bit more friendly, the teams would face off on occasion with epic results. Each team has heroes sporting a range of power sets and abilities but they're not all interchangeable. There are a lot of Avengers who would be a major addition to the Justice League but there are more who simply wouldn't make the cut.
At first glance, Thor seems like the exact type of hero the Justice League would want. He's a being of immense power and strength that can take on any foe, a perfect complement to Superman. The problem with Thor is that he doesn't take direction well, particularly from someone like Batman. He's good with listening to Captain America because he has earned Thor's respect. Batman is a lot more overbearing than Captain America. If the Caped Crusader tried giving Thor directions with his usual tone, Thor is as likely to throw a hammer at him as he is to listen to his strategy. It just doesn't end well.
Wolverine has served as a member of a number of teams in the Marvel Universe such as The Avengers, Alpha Fight, The X-Men, The New Fantastic Four, and The Defenders. The list goes on and on from there. For the most part, all of those team memberships went reasonably well. Wolverine as part of the Justice League would be an absolute train wreck. The core problem is that Logan has no issues with gutting an opponent when the moment calls for it. The vast majority of the Justice League's membership would not be okay with that. Plus, he never stops smoking and you'd have to imagine that the Justice League's base du jour has a strict no smoking policy.
Marvel's iteration of the God of War would be an unlikely addition to the Justice League, in no small part due to Wonder Woman's presence. She's had more than a few run ins with DC's version of Ares so it's unlikely that she'd want to work with a different Ares. It's impossible to blame her on that one. Plus, Ares is also all for killing people so that presents yet another issue for the League. More than that, he rides the line between hero and villain, frequently leaning more towards the side of the villain. While the Justice League isn't above giving a villain the chance to reform, Ares might be a little much for them.
Yet another godly member of the Avengers that the Justice League would probably cut given the opportunity, but for vastly different reasons than Thor or Ares. For the most part, the Justice League are a serious bunch. Generally speaking, Hercules is not. While the Avengers put up with his drunken antics, the Justice League would not be as tolerant of him leaving crushed, empty kegs all over the Watchtower. That being said, seeing him nail Superman in the back of the head with a full keg would be pretty funny, though it would lead to an immediate expulsion. Worth it.
6 Black Widow
Black Widow is an spy and villain turned hero. She's willing to do things other Avengers aren't to get the job done, especially if it mean saving the world in the process. She seems like the kind of hero that would fit in well with the Justice League as she takes her business seriously. In particular, Batman would jive well with her. She's definitely his type of crime fighting partner. The rest of the Justice League wouldn't be so cool with her. Black Widow's outlook on life might not work as well with the balance of the team. In fact, it would most likely lead to a big conflict. She would probably be happier on The Outsiders.
The thing about Namor is that his own teammates can barely stand working with him and they've known him for a long time. Namor is one of the most arrogant, obnoxious and condescending characters in the entirety of the Marvel Universe. That's pretty impressive since Doctor Doom also lives in the same universe. Saying Namor is an acquired taste is an understatement to say the least. It's doubtful that any of the Justice League would be willing to make the leap on teaming with him. Aquaman in particular would have an issue with Namor since he probably isn't looking for competition for the throne of Atlantis.
When you have a lofty title like Protector of the Universe, expectations are high. While a variety of people have had the title of Quasar, the current holder of that title is Wendell Vaughn. His power comes from the Quantum Bands, weapons that allow him to manipulate energy for a variety of purposes including the creation of various objects. Sound familiar? The Justice League already has that power set thoroughly covered by the variety of Green Lanterns on their team at any given time. The other issue is that Quasar's energy constructs are yellow and that is one of the few vulnerabilities Green Lanterns have. It would be like pairing Martian Manhunter with the Human Torch.
3 Human Torch
There are two huge reasons why Human Torch would get bounced from the Justice League. First, one of the core Justice League members is vulnerable to his power set. All Johnny Storm would have to do is flame on too close to Martian Manhunter and J'onn J'onzz would be done. The other problem is that Johnny Storm is kind of a jerk. In addition to being a respected hero, Storm is also notoriously juvenile. He would immediately run afoul of pretty much everyone on the Justice League. That being said, the fact that he would almost certainly kill Martian Manhunter by accident is probably a bigger deal.
2 Scarlet Witch
She's powerful, deadly, and skilled. The Scarlet Witch makes an impressive addition to any team, though the Justice League would definitely think twice about including her in their roster. She has the mutant ability to warp reality using chaos magic. This is a level of power that might concern the League, especially when combined with occasional lack of ability to control those powers. Scarlet Witch has suffered numerous breakdowns that have reshaped reality and caused problems on a nearly cosmic scale. The Justice League would perceive her as a threat that needed to be dealt with rather than as an ally.
The Avengers barely want Hulk around. Hell, the Illuminati of the Marvel Universe once launched him into space, jump starting the Planet Hulk story. That worked out well for pretty much no one. He's barely held onto any level of Avengers status over the years, mostly thanks to his status as a founding member of the team. The Justice League would head straight to fight mode with him, mainly because he'd probably introduce himself by knocking down half of Metropolis. Superman would mistake him for a green version of Doomsday and try to take him out almost immediately. You really can't blame him.