Avatar is astonishing. Beautiful in every sense; somewhat of a Star Wars moment for the new century. Considering 3D has been experimented with in film since 1911, it's hardly a new concept. And, to be honest, I don't see Avatar as heralding the next era of mainstream cinema. 3D is still a gimmick, if very effective here. But then it ought to be, given the time and money spent on the film.
The 3D wasn't the reason that it's being called revolutionary. It's the performance capture, which took motion capture to a whole new level. They recorded the actors mannerisms and facial expressions as they interacted with each other as computer data that drove the CGI segments. Traditionally the actors would have recorded their voices and let a team of CGI artists create their characters but Cameron wanted to keep the human element of the actors performance.
Avatar's true success is revealed when you consider the fact that tickets for Avatar are at least twice what they were when Titantic was screened. When adjusted for inflation, it's in the top 30 highest grossing films of all time, I believe.
There's a million different ways that you can spin box office numbers for movies. If adjusted for inflation "Gone With the Wind" is still number 1. But "Gone With the Wind" also came out at a time before TV, video games & computers. Back than it was common for people to go to the movies once a week or more. Plus it's been re-released multiple times over like 3 generations. On the flip side the American population was much smaller than it is today during most of those days and tickets were cheaper.
"Titanic": also didn't have to contend with HDTV's and rampant internet piracy.
I'd say that Avatar's true success is that in an era of HDTVs, Blu-ray, X-Box360s, Pay-Per-View, pirated DVD's & Netflix it got people who usually stay home and wait to watch things on their home theaters to pay $10 at the multiplex to see it. Plus it continued to post big numbers week-after-week in an era of one-weekend-wonders.