It's about building an audience. Batman Begins had a fairly modest Box office take, but obviously had good legs, and good word of mouth, to where it brought what TDK did in the second installment.
Iron Man at the time was the second biggest opening for a non sequel (now 3rd thanks to hunger games) but Iron man's cultural awareness was pretty limited. I'm about as big of an IM fan as you can find, and I thought the film would do in the 70's or so opening weekend. Obviously it did much better and had great legs despite going up against Chronicles of Narnia and Indy 4 in the following weeks. If it had the luxury as TDK had of weeks of no competition, I think it would have made 400 million.
Spider-man had built-in name recognition and plus after the tragedy of 9-11, I think the country was ready for a movie like that. About heroism and self sacrifice.
However, Avengers is to the point now it's beyond needing someone like Spider-man in it. In fact I think having Spider-man would be a detraction from what the story is. Spider-man was never an Avenger (although I think he was considered a long standing "reserve"), until the New Avengers came out about 10 years ago.
So really I don't see anything holding back this film. I expect it at minimum to out gross all the other Marvel Studio films, but I really think this could be one for the record books. But there's no predictor on that.
Clearly from reviews that have come out already from folks who considered TDK the gold standard have at least put Avengers in that catagory, if not outright on top.