Lets see, the Dark Knight's big success was mostly because of of Heath Ledger's death, it would still be one of the 10 most successful movies at the box office that year, maybe even top 1 like it ended being, but Heath Ledger's death atracted a lot more of interest.
With The Dark Knight Rises you don't have the Joker, Batman's name will obviously bring tons of money but not as much as Heath Ledger, however, with the help of TDK and Inception Nolan's name has groun in the public's mind, the movie will surelly make much money but not as much as The Avengers, and i'll tell you why.
First, Joss Whedon is going to direct this so the movie won't lack character development and good dialogue, the story also seems to be focused, Joss Whedon has proven that he can appeal to both, the masculine and the feminine public.
Then you also have to consider that Whedon has a strong fan base, i noticed that a lot of Buffy fans for example began to watch his other shows just because they had his name, many that aren't really into the Marvel Cinematic Universe thing got interested too.
Well, the director alone will bring some good public, of course it won't bring as much as Nolan's name does, and Serenity proved that, yet both movies have pedigree and an extablished fan base, that has to count for something right?
Then what will trully make Avengers a smach hit, it's a crossover of 4 extablished franchises, Captain america and Thor didn't have sequels yet but they're pratically franchises by their oun as there are in least 2 sequels prepared for each one, both have video game adaptations and both are enjoying a run in comic books for some years already. I heard many saying that all these films, Iron Man, Thor, etc have the same audience, not exactly, sure, many that saw the other movies return because they're all superhero film and some members of the audience know the movies are conected.
I don't know if anybody noticed but Thor got the attention of a lot of women, even more than Iron Man, maybe it was the romance, some like Loki, but what matters is that same public came back for Captain America, and liked it too, for the exact same reasons, well, this time there was not Loki, but there was still romance.
And as a matter of fact not everybody knows these movies were connected, not every body knew there were post-credits scenes, that's why some didn't return for for the other movies, because they tought that was just another superhero movie, now they see the Avengers trailer and see their characters there, and so they will want to see it.
Fan girls of Twilight also seem to like this, with their favourite romance movies gone they will simply switch to superhero movies that have those bits of romance like Thor and Cap, how do i know this? Because i've seen this happen, Twilight saga may be terrible movies but they are still mammoths at the box office, imagine that together with all the comic book fans that are going to want to watch this event.
Not to mention that The Avengers trailer was the most downloaded from iTunes of all time, and the trailer wasn't even that special, i think TDKR trailer was more emotional, yet this only proves the level of Hype there is for The Avengers.
The Avengers may be even bigger than Titanic, and if really, really lucky, it may even surpass Avatar, i only see The Dark knight surpassing Transformers 3 at most, i don't think it can even reach Harry Potter and the Deadly Hallows Part II, a movie whose level of hype was much bigger