Now that the film has surpassed $1 billion worldwide, though, I wanted to weigh in on the issue:
During the comic book boom (late 80s-early 90s, incidentally when I was collecting), the Avengers were NEVER Marvel's signature property: they never occupied the same space in the business side of the Marvel world as their counterparts at DC, the Justice League, even though they were counterparts within their respective universes. Respectively, neither Captain America's or Thor's one title sold as well as the least-selling one of the four that Batman and Superman both had.
Marvel's biggest sellers during that boom were:
- The X-Men (notably Wolverine of course)
- Spider-man (who at one point was starring in five monthly titles: Amazing Spider-Man, Spectacular Spider-Man, Web of Spider-Man, Spider-Man, and Marvel Comics Presents)
- The Punisher (a symbol of the Modern Age of comics and a character long since left by the wayside)
Not so much nowadays
... and big, world-spanning Avengers events (like Acts of Vengeance) were only initiated once Marvel had proven they could do the same thing in previous years with the X-Men (notably with Inferno). Not only were the Avengers never a huge deal, but none of their individual members were either -- except for the Hulk under Peter David and Todd McFarlane (later Dale Keown). But that was more a matter of the talent driving sales rather than the character, and at that point Hulk hadn't been in the team for years. None of the Avengers were cool. None. Of. Them.
Take this cover from The Infinity Gauntlet: the world-stomping crossover that defined much of the Marvel Universe in 1991, with Thanos the Mad Titan taking over everything. (Yes, he's the bad guy at the end of The Avengers.)
Yes, Captain America is in the front, but that's only because he was supposed to be the leader whenever all the superheroes got together and because Marvel was pushing his 50th anniversary that year. The featured heroes on this cover are Spider-Man, Wolverine, and Cyclops. OK you'll say "But Whitleypedia, Scarlet Witch is pretty big too, and she was an Avenger!" Yes, but that's because Scarlet Witch is also a mutant.
Hulk's pretty big, but again, mainly because of the popularity of the talent on his book -- and he wasn't an Avenger at the time. Notice how Iron Man and Thor are tucked waaaay in the back EVEN THOUGH Thanos was always an Avengers villain!
I imagine it took some negotiating to get even these two main X-Men included in IG, given that the X-Men franchise were so busy in 1991 with their "Mutant Genesis" to get involved with Infinity Gauntlet in any meaningful way. Marvel had just started a second X-Men title (creatively titled X-Men), which was the No. 1 selling comic book of all time. Given the current market, this record will stand forever.
8.1 million copies of this thing sold in 1991
So naturally when Marvel starts making movies in the early 2000s, they lead with their strongest properties -- X-Men and Spider-man -- but then by the time they've refined the process of screen adaptions, those two stories are pretty well played: everyone hated X3 and SM3, and were indifferent to Wolverine and X-Men First Class. And The Amazing Spider-Man, regardless of quality, will get overlooked this year between Avengers as Dark Knight Rises. ASM should have been a Thanksgiving release, well after the summer movie season but a month before The Hobbit comes out.
So thanks to the success of the X-Men and Spider-man franchises, Marvel finally had the clout to try something more ambitious: a world-spanning team up on-screen like they'd been doing on the page since the Silver Age. And to do so, they tapped their benchwarmers ... who are now their star players.
A sign that the Avengers have become Marvel's top-tier property (beyond the movies naturally) is the current Avengers vs. X-Men series. When I was a kid reading comic books, the Avengers were NEVER cool enough to share a masthead with the X-Men. But nowadays, on his Twitter feed Stan Lee lists the X-Men last! So I guess this is an example of the last being first and the first being last.
OK second last before "etc."