It's the law

When we're world-building for SF we're not quite as free as it might first appear.  Our world has to be built according to laws of some kind, and then we have to stick to them.  As I'm writing SF rather than fantasy, my worlds have to conform to the known laws of science.  That doesn't mean that it isn't possible to have exotic aliens, it just means I have to work much harder to justify their existence. 

In the interests of story, we generally want our human characters to be able to move about freely when they're on-planet, and that usually means putting them on a planet that's not dissimilar to earth.  But if you think that limits you too much, think Avatar.  In many respects that world was familiar.  It had jungles, an atmosphere, oceans.  But within those parameters is scope to be exotic.  The bioluminescent forest at night was an extension of the bioluminescence some plants on Earth have, but scaled up and with every plant glowing it made for an alien world.  There were horses and birds on that world too, and flying dinosaurs.  Every creature has a similar counterpart on Earth, and yet they were truly unique.  

Pandora has trees too, but while they're recognizable as such, some are much more than mere trees. In the Tree of Souls I see the form of Earth's weeping willow in its arching branches, but everything else is different.  The tree has its own form of sentience, a role as memory-keeper, something no Earth tree can do.  The Na'vi too are recogniseable.  They're a standard humanoid body form, but they're ten feet tall and much thinner in the body than humans,

The rules we set up for building our world will influence the society and culture of the people who live there.  The Na'vi live in the forest, and respect and worship the natural world.  Species who live on the ocean where there are huge creatures would most likely build up legends, and probably some kind of religion, around the creatures.

If there are several different species populating your world, each will have their own culture and moral code.  If they are organised societies they will have some form of law.  What is forbidden for them?  Are they free to choose their friends?  Or is there a strict caste system that prevents free thought?  And what happens when an outsider comes in and shows that their rules don't make sense?

Consistent biology, creature design, and laws and cultures arising out of the environment and what its people do there, allow us to tell a convincing but alien story,