The last 30 mins of both Terminator movies are practically carbon copies, but again, I love bother, and the second more than the first.
Serenity did bugger all for the sci-fi community. If it wasn't for the great series, it wouldn't have been what it is today. It pushed no boundaries, and is actually one of the most loathed sci-fi genres - western sci-fi. You can't criticise Avatar & praise serentiy, when the ships could simply be replaced by horses, and space with the wild west. If you then mention that that's missing the Reapers:
1)They were a minor part in the series
2)They are just as important in Serenity as the connection to Pandora in Avatar.
Take away the Reapers and the connection, and then you have to "meh" films, but still, Avatar is a technological masterpiece, but has great acting, and a wonderful musical score.
There are no connections to the planet in either Heroes (a disaster of a show) or the Last Samurai. That'd be like saying T2 is just another assassination movie if you take away the concept of robots and skynet. Or that Titanic is just another romance if you take away the boat. It's just missing the whole point of the films to find a reason to criticise it. If I was to take away Middle Earth, then LotR would just be another fantasy. But that again is simply not looking at what the film actually is and instead bastardising it.
All your quibbles would have made the film worse. The point was to connect with the one tribe over the unobtainium, not to tour the planet, and yet a few different areas are shown. Plus, who cares why the unobtainium is expensive? That'd just be digressing and getting away from the story, as well as anything to do with Earth. Those ideas would have made the film considerably worse.
How can you complain about Deus Ex machina, yet the two Terminator movies end up in exactly the right settings needed to destroy the "unstoppable" machines?
Thing is, aliens would just be another film if you said it was just Alien meets Rambo. And how can you praise terminator when the idea is a robot assassin is sent back in time? Not exactly a ground breaking concept. Then followed by the same again, yet instead of a human, the robot now protects the target. Again, not pushing the boundaries of plot. Where the magic lies is within the casting, the script, the musical score, and the most important part, the execution. If it wasn't for that, the Godfather, American Gangster and Goodfellas would just be "another gangster story", which I hope you can see where the idea of a movie plot being 100% original isn't actually too important.