Does everyone forget that Dunst made a name for herself in Interview With a Vampire where she got better notices than Pitt and Cruise? That, btw, was a big hit. Jumanji, Wag the Dog, The Virgin Suicides, and Bring it On didn't hurt Dunst's career either. She wasn't A-list, but she certainly wasn't someone looking for her big break either when she was hired for Spider-Man.
I like Emma Stone, but I think there's a bit of overstatement calling her "all but A-list". Easy A was no Bridesmaids at the box office. The Help was a big hit, but people came out of that movie talking about Viola Davis, Octavia Spencer, and Jessica Chastain. Stone has had something like two lead roles, Easy A and The Help, of significance on her resume and one was a moderate hit and in the other she was overshadowed by the supporting cast. Obviously she's been a supporting character in a number of movies with good notices and box office (Zombieland, Superbad, and Crazy Stupid Love for instance), and she looks like she'll become a star, but I don't see how she's somehow a significantly bigger name than Dunst was in 2002. Both are/were known with some hits and critical successes on their resume, but neither is/was a household name.
I think we also should remember that calling people "stars" implies that they stood out in a movie, not that they were in a big hit. Lots of actors are in big hits. The Social Network was a good hit, but how many people came out of the movie talking about Andrew Garfield rather than Jesse Eisenberg, Fincher's direction, Sorkin's dialogue, and, heck, Armie Hammer's Winklevii? I think the case can be made that Tobey Maguire coming off lead performances in The Cider House Rules and Pleasantville was a bigger name than Garfield is now. TASM will be Garfield's first lead role in a movie of significance while Maguire had a couple at that point. I don't think there's much of an argument that Maguire was much more established as a leading actor at the point Spider-Man came out, since he'd actually been a leading actor in significant films.
Which isn't to imply that either Maguire or Dunst were "A-list" when Spider-Man came out, but there's some rationalizing going on here and cherrypicking of points that are convenient and ignoring of data that's inconvenient. To me, there's no question that Dafoe, Maguire, and Dunst were a bigger collection of stars than Rhys-Ifans, Garfield, and Stone. There might be a point that Leary, Sheen, and Field make up some of the gap over Simmons, Robertson, and Harris, but I'm not sure how much the supporting cast matters to the general audience.
Heck, why does it even matter? In 2002, Spider-Man had as good a cast as any superhero film that it was competing against. In 2012, Spider-Man's cast has virtually no star power compared to the likes of The Avengers and The Dark Knight Rises. As for recent superhero films, I'd certainly put the cast of TASM in terms of star power behind any of the Iron Man films, Thor (solely due to Natalie Portman who's actually an arguable A-list actress), and the Batman films. Maybe even behind Captain America which had Hugo Weaving. There's no novelty to Spider-Man being on the big screen and there's plenty of superhero competition in 2012. What's TASM doing to sell itself in this field that's making the general audience take notice?
Ultimately I think Spider-Man will do fine, especially worldwide, because it's Spider-Man, but there's very little buzz about the film other than on superhero movie boards. Something is missing from the marketing.