1) I'm not trying to say IMDB is perfect. It has obvious flaws (starting with Pulp Fiction, Shawshank Redemption, and, yes, The Dark Knight being anywhere near the top 10). But you don't seem to understand their ratings system - it's not just a simple average:
"IMDb publishes weighted vote averages rather than raw data averages. Various filters are applied to the raw data in order to eliminate and reduce attempts at 'vote stuffing' by individuals more interested in changing the current rating of a movie than giving their true opinion of it."
They also only count the votes of regular voters when calculating the top 250 list.
2) According to RottenTomatoes, The Dark Knight received a 94% positive review score, with an average rating of 8.5/10. For comparison's sake, No Country for Old Men received a 95% and 8.6/10, There Will Be Blood a 92% and 8.4/10, and The Departed a 92% and 8.2/10.
"Christopher Nolan's “The Dark Knight” is a haunted film that leaps beyond its origins and becomes an engrossing tragedy."
-Roger Ebert, Chicago Sun-Times
"The haunting and visionary Dark Knight soars on the wings of untamed imagination."
-Peter Travers, Rolling Stone
"This film is not only one of the year's best; it may well end up as the finest of 2008. At the very least, it deserves consideration for Best Picture and Best Director, along with the expected Oscar kudos for Ledger."
-Peter Howell, Toronto Star
3) Many film historians and theorists have questioned the position and acclaim that our culture affords Citizen Kane. The primary reason it has such vaulted status is not because the of film's inherent merit, but because it was stylistically and thematically lightyears ahead of the rest of the American film industry; it was a modernist film released in the classical stage of American filmmaking. This is why respect for the film and Orson Welles grew so much over time: as the industry progressed, everyone noticed that Citizen Kane had been a mile ahead for decades. But there are many that have, and continue, to question the actual quality of the film (within the context of discussing the greatest films of all time). I bring all this up because many people do tend to name drop the movie as a measure of excellence, even perfection, without understanding the contentions over its status.