It makes perfect sense if you look at trends in movies over the past few years. Except for the occasional masterpiece (Dark Knight), action movies have largely reverted to pre-Die Hard explosion fests with poor CGI, derivative music, and WWE stars who, as it turns out, aren't the best actors in the world (who knew?). Add to this all the aping of actually good filmmaking techniques (the Wachowskis' bullet-time, Paul Greengrass' and Christopher Nolan's use of the shaky cam) by hacks like Michael Bay, Roland Emmerich and a long list of others whose names you'll probably never hear again, and the result is an audience that hasn't been visiting theaters as much lately FOR A REASON. More than anything else, Avatar was a phenomenon of one of the big boys returning to show us how it's done.
Also, word of mouth spread like wildfire, and the overwhelming majority of the films detractors (ESPECIALLY on the Escapist) haven't even seen the film (use the search bar, you trolls!).