Which was more groundbreaking in its time: original Star Wars (1977) or Avatar (2009)?
As of this writing, the top two answers are:
Definitely Star Wars.and
Avatar is nothing more than an expensive tech-show, with no underlaying story or idea whatsoever; so that's not a fair fight.
Now let me be clear, I am a Star Wars fan through and through. When I was about 12 I played a Twi'leki in the (pre-Wizards of the Coast) Star Wars RPG. My shining moment was when I shot down a TIE fighter from the back of a speeder bike (TIE fighters aren't very aerodynamic... It shouldn't have been in an atmosphere, so lots of penalties, etc.).
With that fan-boy armor donned, I have to say that I very much disagree with the statement that "Avatar is nothing more than an expensive tech-show, with no underlaying (sic) story or idea whatsoever," at least as compared to Star Wars.
Actually, I would argue that Star Wars and Avatar are complementary story archetypes. Star Wars is essentially a classic magical story in a sci-fi setting. Avatar is in a lot of ways a sci-fi story in an old-fashioned naturalistic setting.
Now, if you can find someone who hasn't yet seen Star Wars (and they do exist) and show them A New Hope, Empire, and Jedi, you'll probably be surprised at their reaction. Once you remove the fine glaze of nostalgia that most of us have for the original trilogy, you'll see how bad parts of these movies really are. As seen through novel eyes C-3PO is an incredibly terrible character. While it's not quite as annoying as Jar Jar, 3PO is at least in the same ball-park. And Luke is frustratingly whiny (whingy, if you're a Brit). Seriously he's Shinji-level whiny.
Granted, the technology used in creating Star Wars was amazing for the time. And the story is definitely engaging. When the dialog isn't Lucas's, it can be poignant and even moving. I still believe that Han's "I know" response to Leia's "I love you" is probably the best in film (followed closely by Spike's "no you don't, but thanks for saying it," in response to Buffy's "I love you"). The story is a well-told one overall but it is improved by being set in a novel (for the time) sci-fi world.
Avatar's story is derivative, too. But it's also in a novel setting. I mean, you've got the whole transfer-of-consciousness tech that is used in a really cool way. The only other place I've seen idea fleshed out is in Richard K. Morgan's novels about his Takeshi Kovacs character.
Step away from your nostalgic, pre-curmudgeonly late 20s/early 30s self and look at Avatar through the eyes of your 9-year-old self. Up until Avatar, all you'd have seen are some crappy 3D gimmicky movies. But then Avatar takes the tech to a whole new level. What 9-year-old doesn't want to be a giant, strong, fast, tough, gun-toting cheetah monkey? That's just cool!
Now, I know it's all the rage these days for neuroscientists to try and connect some pop-culture thingy to the brain somehow. I could say that nostalgia reinforces pleasure circuitry through relived memories of a simpler, happier or some such, but that would be a bunch of crap. All I know is that nostalgia can be a very powerful lens through which we observe the world around us.