I feel like Marvel taking place in a "real world" setting is an advantage. I don't mean it's
realistic, but rather it sets itself as being our world
if it were a superhero world. The locations are real, with the prime example being New York City, and people being born in real places. DC primarily relies on fictional places like Gotham, Metropolis, Star City, Central City, Bludhaven etc. even when not as fantastical as something like Themyscira or Atlantis. Marvel
does have fictional locations, yes, that isn't being denied, but those are treated as the exception, and when present are more fantastical like Wakanda, Latveria, Attilan, Atlantis, Madripoor etc.
Those are the exception, and not the rule.
I think just having the Marvel characters come from actual real-world places with defined locations helps to better sell them as real people instead of a fictional city of a vaguely-defined location.
That brings us to the next point, the fact that Marvel heroes have stood apart from DC heroes as being more like real people with real struggles, and just as much focus on what's behind the mask as there is the costumed hero.
Many heroes have a defined struggle which the audience can relate to in some way, yet prove to be good people through it all.
Take for example:
- Spider-Man has a sense of realism in that the dead stay that way and status quo changes have consequences from one arc to the next (Clone Saga and OMD notwithstanding), and Peter struggles with many of the mundane problems we do.
- X-Men and the mutants go through prejudice that is like that of real life minorities, racial or sexual or any other (albeit in a more fantastical way). Hence, they related to so many people and why they became so popular.
- The Fantastic Four are a family that bicker and fight, yet still love each other, just like any real world family.
- Hulk with his psychology issues and dealing with inner demons.
- Iron Man struggles with alcoholism, and it's played realistically.
- Daredevil being blind and having a troubled past while being a devout Catholic.
- Jessica Jones having PTSD from a very bad experience.
- Quicksilver's powers are a hindrance as much as a help.
- Hawkeye having an abusive dad and finding a better path.
and so on.
There's also the fact that Marvel, from the beginning, set itself as being part of a living breathing world. That's why it's so common for heroes to meet each other outside of special crossovers while also going about their own lives, they fight villains normally associated with someone else while having their own rogues, they have supporting casts migrate between different characters (such as Iron Man getting key Spider-Man character Mary Jane Watson as a PA for Stark Industries), and that many worldbuilding elements are present throughout all the books. Events that happened somewhere else can be relevant to a different story, and it's common for books to reference each other (often with captions to tell you where to look).
This began because Marvel was centered primarily around two creators: Stan Lee and Jack Kirby. Yes, there were others like Bill Everett, Larry Lieber, Don Heck, and of course, Steve Ditko, but they were the core. It was both for reasons of differentiation (DC not having a connected universe then and Marvel selling itself as the hot alternative), and practicality (because it's just easier to swap around concepts with two guys are in charge versus a whole company). That said, they wanted their world to feel alive, where anything can happen, and where there's one connected continuity that affects everything. DC actually had to catch up because Marvel's formula was so successful, but I still prefer how Marvel does it overall.
In DC, you have the Trinity of Superman, Batman and Wonder Woman. However, you can't really do that with Marvel. Fans have recently tried that before with Iron Man, Captain America and Thor, but that is an extremely spotty way to do it. Marvel is just too widespread for it, and making them the Trinity leaves a bunch of massive elephants in the room. They can be the Avengers Trinity, but not the Marvel Trinity, in my book. It'd be best described as four pillars: Fantastic Four, Spider-Man, X-Men, Avengers.
That's my take as to the advantages Marvel has over DC.