...a MacGuffin literally called Unobtainium. Which is literally mentioned once almost as an aside, before it's promptly forgotten about.

That's not true at all. The valuable property of unobtainium - that it is a room temperature superconductor - is shown constantly, and is directly relevant to the final battle. It's why the Na'vi can contest the RDA in the air - because the same thing that makes the location valuable also produces terrain that handicaps the RDA's sensors and punishes a less maneuverable flier.