It's amusing to watch how the response and enthusiasm Dark Knight has shaped since the film's release. Part of these proclamations that this film suddenly deserves a place next to the best films of all time and can even rival Citizen Kane might have to do with where we are in terms of Web 2.0 and the state of film criticism.
Many moviegoers have long been aware of the massive expanse of film history. Ten years ago, the thought that for approximately 100 years, masterpieces have been coming out and the average moviegoer hasn't even seen or heard of a great number of them, prevented the moviegoer from thinking that that amazing picture he just saw must have been the greatest film of all time or close to it. When a film comes out that people think is great, they are generally aware that as great as this film was, the odds are that a number of films they haven't seen were better, simply because there are so many.
The increase of film retrospectives (the AFI 100, for example) and distinguished film historians, successfully solidified that amorphous category of "great films from the old days that i haven't seen" into a concrete, slightly flexible block of films with names and faces like Rebel Without a Cause, On the Waterfront, Graduate, Citizen Kane, All About Eve, etc. which continue to be solidified by subsequent "Greatest Films Ever" lists produced by every major publication (Entertainment Weekly, Time Out, Guinness, Premiere, Rolling Stone) that deals remotely with films that echoes the original choices.
At the same time, this community of people doing the film retrospectives and putting in the thought about what the great films are has become far more interactive. In the past five years there has been a major growth of people with film blogs, registrations on websites like imdb.com and flicker. This might be why there is some sort of shift and enthusiasm to challenging the notion that a recent film is better than all the classics, because as one of the people proclaiming its praises, you are taking some small degree of ownership over that film. I think the Dark Knight's brief appearance on top of the imdb.com's 250 and it's potential chances to knock off Titanic (the last time this sort of phenomenon happened) at the box office, alerted the film community that they could seize this film and proclaim their own "Citizen Kane."
The truth, however, aside from whether the film is Citizen Kane is not (it is not because it doesn't really revolutionize the way filming is done unless you count shooting it in IMAX or the marketing, which is coincidentally what all the people who are proclaiming it so great are slightly influenced by) is it's, by definition, impossible to tell if a film is worthy of being a classic until years down the road. That's why The Searchers, Hitchkock's best work, Citizen Kane, Nashville, and 2001: A Space Odyssey, and Raging Bull all failed at the Oscars: because their historical value wasn't apparent at the time, and why would it have been?
To people watching Citizen Kane at the time, the way it's cinematography and story telling methods would be used later by so many other film makers would not have been known in 1941.