I love science fiction. There. I admitted it. Star Trek. Star Wars. Stargate. Star-whatever. I love it all. But I do have an issue with what I call the internal coherence problem.
Internal coherence means that no matter how odd the rules of the game may be - i.e., no matter how fantastic the sci-fi premise is - once those rules are laid out, the plot must adhere to those self-evinced rules once they are set. In other words, the plot must be coherent with respect to whatever rules were used to define its own universe. So make up the rules and make them as crazy as you like, but once they are made, you have to follow them. And if you fail to set out rules, then your plots must adhere to basic logic, even if the rules of physics are set aside for the sake of fun. Let's take some examples.
-Language. No surprise I am starting here! I have no problem with Star Trek's universal translator. It's a clever plot device, the linguistic equivalent to the very convenient transporter technology of that same show. And at least it helps to address the problem of why everyone in space appears to speak perfect English. But it should follow basic logic in its approach. For example, it either works or it doesn't: it is confusing when 90% of the time it is on automatic and translating everything people say, but then suddenly someone can say a word in another language, but that one word isn't translated.
One area where it shouldn't work, no matter how clever these 24th-century programmers are, is with a completely new language. I don't care how advanced your technology is, me saying to you, "Hello, I greet you in peace, and by the way, your starship is double-parked in a handicap zone and is about to be towed" does not provide sufficient information for you to decipher my entire language. You can't deduce "I'd like to order a pepperoni pizza" from someone saying, "Hey, I love your cool starship".
-Space. You'd think one no-brainer area for people writing about stuff happening in space would be, well, space. I guess we're just so accustomed to operating in two dimensions, that we just forget how much space there is in space! For example, sometimes a ship is 'surrounded' by enemies ahead and to their flanks. It's SPACE, not the ocean. Just go 'up' or 'down' instead of forward. (These terms are used loosely, because of course there is no 'up' or 'down' in space...there's no fixed body against which to measure something as 'up' or 'down'....which is sort of the point here.) The other major problem is sound, which can't exist in the vacuum of space; but I give them a pass on this one: you need sound as a dramatic effect.
-Planets: Again, if you're writing about outer space, this should be something you get right pretty frequently, but alas, no. Some oddball things about planets in sci-fi: 1) Why is it that despite these planets often being at least as big as (if not bigger than) Earth, everyone always crash lands in the same place? What are the odds that two ships from space, landing independently at different times and with no predetermined plan, would land in, say, Tuscon, Arizona in the US? Blindingly little chance. 2) Why is is that every planet save ours has a world government with a single capital for the whole planet, and everyone speaks the same language across this world? We have ca. 200 nations and speak over 6000 languages...why do aliens just have the one government and the one language?
-Aliens. Why does everyone is space look human, only with funny noses or foreheads or ears?! Strange as it may seem, this is actually one that doesn't bother me as much as the others due to a variant on the theme of the so-called Law of Mediocrity*: evolution will often find the same answers to the same or similar questions. So, if billions of years of evolution lead to the dominant species being a (relatively) big-brained biped with a nose, two eyes and two ears, then it is not unreasonable to expect a similar model if circumstances are similar elsewhere. And since our definition of intelligent life presupposes an Earth-like planet, it may be reasonable to expect intelligent beings from space to be not entirely different from humans. In theory. Possibly. [Insert your own generic long string of caveats here, because we have no way of testing this idea!]
-Matter matters. For reasons I don't understand, one favorite plot line across many sci-fi universes, is the 'matter-less being' plot. Example: there's an accident (or some crazy device involved) and characters X and Y become invisible to everyone and can pass through walls. Fine...it is sci-fi after all. But then, how do they stay standing on a floor? If gravity and matter are irrelevant to them, what affixes them to the floor even as they can walk through walls? Shouldn't they be able to dive through floors as easily as they walk through walls? Very confusing.
*Sigh* There, I feel better now. Now I can go back to watching more cheesy sci-fi.
*It's not really a law.