Viability

Does this make sense?
This is a make-or-break question for most genres of speculative fiction. If the worldbuilding doesn't hold up logically, it can completely undermine a narrative as readers roll their eyes while struggling to suspend their disbelief. Often, viability failures are the result of insufficient research or poorly explored premises. 


When Things Go Wrong

Aliens capable of interstellar travel invade the Earth. 
 
They do not wear clothes or armor. 

Water kills them.  
 
70% of Earth's surface is covered in water. 



Signs is a staple on lists like "Worst Alien Invasions" &"Dumb Moments in Science Fiction."
Implications

Consider the global ramifications of your speculative elements. 
When you change something about the way the world works, it's important to think about all the ways that change might impact your world. Exploring the full implications of your speculative elements can make your worldbuilding more vivid and believable. Failing to do so can result in viability issues. 




Example: If every household has a 3D printer capable of printing anything, Walmart is probably out of business. In fact, retail commerce as we know it is over. 

Example: If medical breakthroughs make humans immortal, what is the new retirement age? Do people who've lived for several hundred years treat people in their forties like children?