WATER, WATER EVERYWHERE...

Good old H20 is essential to life (as we know it!).  Luckily, it also seems to be pretty common in our Solar System, at least in its solid form of ice.  Even liquid water may be more common than we once thought, with oceans on Europa, Enceladus and Ganymede.  (https://www.sciencenews.org/article/aurora-shift-confirms-ganymede%E2%80%99s-ocean)

So my pet peeve?  SF writers who have aliens traveling light-years across the galaxy to steal our water. (By "our" water, I mean the Earth's water.  I leave to the reader the exercise of determining whether humans, vis-a-vis aliens from another star, "own" the Moon's ice, or Saturn's rings).

Why take our water?  It's at the bottom of a gravity well and surrounded by angry primates with nuclear weapons and a self-destructive streak.

There are things that invaders might want: heavy elements, which tend to congregate in rocky inner worlds, biological products that are unique to Earth's biosphere, etc.  Or they may just want to eliminate potential future competitors (http://news.discovery.com/space/alien-life-exoplanets/space-invaders-unlikely-for-now-130214.htm).

Just don't use water as your crutch, writers.  Pick something unique!