Defining science fiction: Hard vs. Soft

Have you ever wondered if the science fiction you read (and perhaps write) is "hard" or "soft"? Use this handy guide to help you understand the difference between hard science fiction and soft science fiction.

Story element	Hard science fiction	Soft science fiction
Story is set on Earth, in the present day	Today's technology offers many opportunities for dramatic tension	Are you kidding? Where's the fun in that? BORING!
Story is set on Earth, in the future	Mapping out a logical forward evolution from today's technologies sets a plausible framework	Spaceships! Unitards! Laser guns! PEW! PEW!
Story is set on another planet, either in the near future or distant future	Only if the planet a) was originally Earth-like, b) was terraformed, or c) is home to a closed-environment base with breathable air.	Sweet! I hope the alien dancing women have green skin and loose morals.
Main character is human	Descendant of a diaspora from Earth	Who cares where he came from?
Main character is an alien	OK, but even if he/she/it is able to tolerate the temperatures and humidity levels that humans do, he/she/it is unlikely to be able to comfortably breathe the same mixture of nitrogen/oxygen humans do. He/she/it will require a gas mask of some kind.	Is she hot?
Faster than light travel is not possible.	Of course it's not possible. Why would it be?	"Not possible"... what does that phrase mean, exactly?
Faster than light travel is possible, but arduous	*sigh* Fine, but it has to be a result of some fundamental aspect of the structure of spacetime of which present day science is as yet unaware. Wormholes, quantum foam transsubstantation, ancient alien node-point teleporters, etc. The workings of these MUST be explained in detail.	"Warp 4, Mr. Sulu."
Faster than light travel is not only possible, it is easy	Have you ever even opened a physics textbook?	"It's the ship that made the Kessel run in less than 12 parsecs."
Main plot: Hunt the MacGuffin	This can certainly be done in a science fiction context. MacGuffin stories are all about character interactions, anyway.	I love MacGuffin stories!
Main plot: Rescue the princess	Sure, but where is the science fiction element in this? Is the princess a prisoner of some advanced technological enemy?	Does the princess have green skin, a fast metabolism and an open mind?
Main plot: Avenge the death of a loved one	Look, you can't just bolt on some laser pistols, spaceships and robots and call it science fiction. That makes the sci-fi elements just superficial window dressing. Where is the consideration of the ramifications of technological change?	Revenge? Sweet! Can I use laser pistols?
Words expended on infodumps explaining the technologies present in the story	As few as possible, but some infodumping is unavoidable. In order to establish plausibility, some technical detail has to be presented.	None. (Thank God! Infodumps are tedious.)
Readership	All right-thinking science fiction fans should insist on a scientific and technological underpinning that is consistent with the known laws of physical, chemical and biological science.	Hey, Hard Sci-Fi dude? You're getting in the way of a good story. PEW! PEW!

I hope this clears things up for you! Did I leave anything out?

UPDATE: For a much less tongue-in-cheek definition of hard science fiction, see my post on this over at Write Anything.