4The All-in-One Instant Plot Fixing Tool
The heroes are running out of time and they must do something NOW in order to save the day. All of a sudden, a space version of a Swiss Army Knife appears (e.g. Tricorder, main deflector dish, R2D2) that happens to be able to do just what is needed to save the day... quite tidily, in fact.
Even if it can't do exactly what the heroes need, it can usually be easily modified to do that exact thing, even if the thing it needs to do is completely made up. A common variation on this is the sensor that senses everything and then senses the side effects of everything else, even things the designers didn't know existed.
The writers of Dr. Who flaunt this idea with the "sonic screwdriver," creating a running joke that it can do anything the plot needs it to do.
Why They Should Stop:
Whether it's epic poetry or sci-fi, the whole fun of a dramatic adventure is watching how the characters use their courage, wits and creativity to get out of these jams. That all gets farted out the window when you realize that the little droid they've had with them the whole time has the magical power to make any machine in the galaxy do exactly what they wanted.
It also creates logical holes all over the place. For instance, once you've shown a character using his phaser to tunnel through a mountain, you immediately think back to every time a character has ever been trapped in a room, and wonder why he didn't do the same. And why would even the most secure doors in the Star Wars universe have locks that can be "picked" by random repair droids?
It's the kind of lazy "get out of plot trouble free" card that we wouldn't tolerate for a second in a story set in the here and now (MagGuyver explained how he hotwired a car with bubble gum, damnit!).
As for the sonic screwdriver on Dr. Who, in the original run on BBC, the producers forced the writers to break the sonic screwdriver to make the show more interesting. The writers had become so accustomed to using it as a crutch that they only agreed to do so because they thought they were going to be able to write another one back in (by the way, it wasn't until the new relaunch of the series that it was actually used to drive screws).
For a corollary to this, also see the "Tim Taylor" rule: if a part of the ship isn't getting the job done, just divert more power to it! More electricity makes every device work better than it was designed to! Just try it with your TV! Of course, if it doesn't work, you could always just reverse the polarity.
3One Planet, One Culture... Except Earth
Our intrepid space explorers make contact with a new planet named Generisis III (cool planets always have numbers) and make contact with the sentient species, the Latexx Foreheadians. Assuming peaceful contact is made, the heroes will be spending a lot of time learning about the Foreheadian culture, language, religion, and all other aspects of Foreheadian life.
But wait a minute. Earth has hundreds of languages, cultures and religions. There's not even a single culture which has "dominance," and the most common trade language (English) is barely spoken by half the planet. So why in sci-fi do we still have races who are "warlike" or "profiteers" that live on a "desert planet" or an "ice planet." Apparently all civilizations eventually just consolidate everything to avoid confusion?
Babylon 5 did an entire episode on showcasing how each race has only ONE religion (to their credit, they say "dominant"), then the following scene is tacked onto the end, drawing full attention to the fact that Earth, unlike the rest of the galaxy, has religions and cultures shotgunned all over the landscape.
Why They Should Stop:
This is a lazy leftover from other genres of writing. Planets in science fiction tend to serve the same purpose that kingdoms, nations or towns do in other fiction. Writers keep it that way because it's easier to write about a new planet if you treat it as if it only consists of five square miles and 50 people.
Even Star Trek, which is supposed to be all about enlightened understanding of these new cultures, still flies their ship from one two-dimensional, monolithic planet to another. If they find some planet with multiple races, it's always to teach some basic lesson about racism, and always for the purposes of that one specific episode.
Once again the genre that's supposed to be expanding our horizons, manages to make the universe seem like a very simple, boring place.