THE GOOD

• The visuals were excellent overall. What really made it was the incredible attention to detail. There were an amazing number of animated elements that made it look more real.
  • The vistas were stunning, eliciting the sense of wonder that draws me to science fiction.
  • The CGI characters were very good. Certainly past the Uncanny Valley (if that could be said to exist with human-like aliens), though if you look closely at certain times you can see that the physics of their movement wasn't quite right.
  • The 3D was OK. There were a few scenes - particularly those with floating debris in the extreme foreground - where I felt like swatting the debris out of the way.
• Decent worldbuilding - We get to see many aspects of the Na'vi culture and Pandora itself.
• Sets a good example for how cool a military SciFi film could look. Those mecha-robots looked they jumped right off the cover of a Warhammer book.

THE BAD

• Clichéd - Maybe I've just seen too many films that feature an Evil Corporation. (Sigourney Weaver's presence only reminded me of The Company from Aliens.) And films about ravaging the land (with its accompanying New Age soundtrack). And the hero who disobeys orders to fight for what's "right". And the friends and foes who join together in the cause. And the inevitable love interest. And the inevitable confrontation with her betrothed. Etc.
  • Two-dimensional, archetypical characters. Sympathetic hero? Check. Tough Military superior? Check. Heartless corporate suit? Check. Jealous boyfriend? Check. I had hoped that the wheelchair-bound Sully getting his Avatar legs would be a bigger part of the film and would lend the character some depth. It wasn't and it didn't.
  • Unobtainium? Really? I get the in-joke, but the name is too much a part of that joke to skip past it without wincing.
• Predictable - Every single plot point was visible long before it actually happened.
• Awkward dialogue. "Shut your pie-hole?" Seriously?
• Bad science - Ignoring the fact that much of the science behind the Avatar project was a lot of handwaving, some things were just downright silly:
  • I get that Pandora's gravity is less than that of Earth, but what's up with floating mountains? I mean, the planet has working waterfalls and floating mountains?
  • Near the end of the film, the Major falls from a ship from very high up. I don't care if he was in a mecha-robot - the impact would have killed him.

BOTTOM LINE: Excellent visuals but it has lackluster story. It's on par with the Star Wars prequels; great eye candy, but not much more beyond that.

Our Galaxy is called the Milky Way. There is no such thing as a "13,000 year cycle of the Sun going through the center of the Milky Way" (or of any other galaxy, for that matter). The Sun - the entire Solar System, actually - revolves AROUND the center of the Milky Way, but this orbit is estimated to take about 250 million years. I consider it unlikely that the Mayas knew those details.

The above is an excellent answer, as far as it goes, but we can have a little more fun figuring out where this notion might have originally come from before it got twisted.

From Earth's vantage point, it's true that the Sun crosses the galactic equator in a cycle. However, it's not a 13,000 year cycle, it does this crossing twice each year, and this year one of those times happens to be in late December (the other is in late June)... near the time of the Winter Solstice, in fact.

It's also true that the time of the crossing drifts very slowly... the crossing points happen on the solstices just about every 13,000 years (an entire cycle takes about 26,000 years, and there are two crossing points, so Crossing Point A lines up with the Winter Solstice in Year X, and then in year X + 13,000, halfway through the cycle, Crossing Point B lines up with the Winter Solstice). It's certainly conceivable that someone might hear that and take away from it "the Sun goes through the center of the galaxy every 13,000 years". This isn't true, but it's a reasonable mistake for a layperson to make.

Now... is it plausible that the Mayans could have known this? The Mayans wouldn't have known what the galactic equator wasexactly, of course, but they would have known what the Milky Way looked like, and if you draw a line kinda sortathrough the center of the Milky Way as it appears from Earth, you get a fair approximation to the galactic equator. Even if you're not horribly accurate about it by modern standards, you still get a line in the sky, and the Sun goes through that line twice a year, and that crossing happens on the Solstices in cycles of just about 13,000 years. Never underestimate the precision of measurements made by what amounts to a priestly caste whose living, and in fact lives, depend on them knowing pretty exactly where the Sun is going to be on such-and-such a date (if you're claiming a direct line to the gods, and you tell the king that on his birthday the gods will cause the Sun will rise just over the temple as seen from his palace, and he gets up early to see it and watches it rise over the outhouse instead, you're not likely to be alive to tell him what it's going to do next year). I think it's reasonable that they could have come up with the 13,000 year figure.

However, this has gotten corrupted by doomsayers into "the Earth passes through the galactic plane periodically." The implication here is that the galactic plane is some sort of magical or at least dangerous place to be, for whatever kind of handwaving reasons the person making the claim feels like throwing out there.

It is true that as the Solar System orbits the galactic center, it does have to cross the galactic equatorial plane at least twice during that orbit, but it's not going to happen any time soon: the Solar System is several dozen light years "above" the galactic plane at the moment, and is actually getting further away from it rather than closer to it.