Before I dive in, let me first address those who aren't really interested in Avatar and don't intend on seeing it. You may not think you give a shit about a movie starring tall blue cat people, but I assure you, you will. You don't have to be a big sci-fi/fantasy fan to enjoy this. Avatar is for anyone who enjoys movies, period. It's a ride, it's an event, it's cool, and it's fun. You're gonna break down eventually anyway after everyone tells you to see it, so you might as well just save yourself the trouble.
Now then, two things to talk about are the technology/special effects of the movie and the story of the movie itself.
Much has been said about the hundreds of millions of dollars and years that went into making this movie, how director James Cameron wants to be an innovator, etc. I've seen advertisements for the film that exclaim "Avatar will change the way you see movies FOREVER!" Which is pretty much the most ridiculous statement you can make. But you know what? It's kind of true, in a way. Avatar is gonna make 3-D a big deal at the movies.
3-D has already started to creep into modern movies. The animated movie Beowulf, which I caught a few years back, was the first new 3-D movie of its kind that I've seen...that's actually a real movie, not like Under the Ocean 3-D or whatever. Up until this point though, it's generally been limited to animated movies and cheesy horror flicks (My Bloody Valentine 3-D, The Final Destination). This is gonna be the first modern 3-D movie that everyone and their mother will see. And it's damn impressive.
Avatar takes place on a exotic alien moon called Pandora. It's a jungle environment with all sorts of various beasties out to get you...and it's quite beautiful as well. There are mountains that hover in the sky and flying glowing jellyfish and of course, the aforementioned blue cat people, known as the Navi.
There are action sequences involving flying, falling, swinging, etc. that are made much more immersive with the use of 3-D and a sense of depth. What it all boils down to is that they created this fantastical alien moon, and the fact that it's right in your face via the 3-D, and beautifully rendered as well, actually makes it plausible enough for you to feel like it's real.
None of this would amount to much, however, if the story was shit. It's not.