Chekhov's Gun: Virtually all objects everywhere. Except the useless ones. Note: which ones are useless changes a bit from game to game. You're welcome.
Clingy MacGuffin: The Thing That Your Aunt Gave You That You Don't Know What It Is. But it's a good thing you couldn't lose it... even when you're not who you were—or where you were, for that matter—when you lost it.
Doublethink: "Intelligence" is defined as the ability to do this, and the only way to enter Marvin's room is to demonstrate that you have intelligence. Appropriately enough, you ultimately accomplish this by physically removing your common sense, allowing you to carry "tea" and "no tea" at the same time.
Easter Egg: A couple of the footnotes have nothing referring to them; you'll only read them if you're specifically going through all the footnotes.
Empty Room Psych: "You can't hear anything, see anything, smell anything, feel anything, or taste anything, and do not even know where you are or who you are or how you got there" seems to be the narrator's catchphrase. If you wait long enough, one of the senses will be eliminated from the list, and using it will help you quite a bit (even if it seems useless at first.)
The Engine Room, at least at first. The reason being that the game is still sulking after you insisted on going into the room against its recommendations. After you pester the game with "look" commands for a bit, it finally relents and lets you see what's in there.
There are a few rooms or areas that have no use whatsoever, except as window-dressing, and don't even need to be passed through on the way to somewhere else (like the walkways around Arthur's home, or the very nice rooms in the flat where the party takes place—except, of course, the room you start in, which is where all the action takes place).
Expospeak Gag: Some items are given a name that's more complex than the everyday name. Top billing goes to the aspirin tablet in the beginning; it's called a "buffered analgesic".
Footnote Fever: The footnotes are all accessed by putting in "Footnote 1", "Footnote 2", etc. Because of this you don't have to wait to be prompted with one, you can read them all whenever. After a certain number it starts telling you there is no footnote for that number, and a little past that you get a note saying "Reading all the footnotes is fun, isn't it?"
Friendly Enemy: The game itself was programmed to emulate this mindset.
Game: Why not? Your situation appears quite hopeless.
Game: Not surprised.
Gay Option: The game recognizes commands to kiss and enjoy characters of either gender... but it just tells you that this isn't that kind of game. Sorry, slash fans.
"This is family entertainment, not a video nasty."
Guide Dang It: On a couple of levels. For one thing, many of the puzzles have no clues except for the in-game Hitchhiker's Guide, and the articles they're found in are sometimes something no one would think to look up on their own ("Brownian Motion"?). For another, said puzzles are often Nintendo Hard anyway, and if you mess any of them up, the game will become unwinnable. Then again, reading the books (including the sidenotes) will help with some of the puzzles.
Have a Nice Death: One of them goes on for several screens, reacting to whatever you try to type with "You keep out of this, you're dead." Another one reads:
Left Hanging: The game ends when you disembark on Magrathea. Plans for a sequel fell through due to many factors.
Logic Bomb: Until you are able to aquire a cup of tea, your inventory will always have the item "no tea". You can't drop it, you can't do anything with it. However, once you fix the drink dispenser on the Heart of Gold, you can acquire "tea", and "no tea" will be dropped there. For the reasons listed under Doublethink, you need to have both.
Nice Job Breaking It, Hero: When you issue a command the game doesn't recognize, it makes a note of it, then later informs you that when you typed in that command, you inadvertently plunged a far-distant galaxy into a terrible war, which caused the races involved to unite in an intended war of genocide against your home planet. Later becomes a Brick Joke (and opens up a few additional ways to die—but also is required to pass a significant puzzle in the game.)
Nintendo Hard: Oh, so very much. This game has been known to cause grown men to cry at the mere mention of the phrase "Babel Fish."
Press Start To Game Over: If you fail to get out of your house after the first few moves, you die. Thus setting the tone for the remainer of the game.
Press X to Die: It is not a good idea to type "escape" when you are imprisoned, strapped down or lost in the dark. It works and is fatal.
Reality-Breaking Paradox: If you end up inside your own head again after you have removed your common sense, you'll kill yourself because this time you're normal-sized. It takes a few turns for the death to catch up to you as the player.
Supporting Protagonist: It can be helpful to ask the game at certain points, "Who am I?" Because the answer is likely to change without notice.
Temporal Paradox: You'll revisit the same events as different people, alter the past through things you haven't done yet, and just generally give causality a good pantsing.
Unexpectedly Realistic Gameplay: Giant parts of the game. You have a headache? Take an aspirin. After taking it out of your dressing gown pocket. After opening the pocket. After putting the dressing gown on.
Unreliable Narrator: As if the game wasn't hard enough already, the narrator will sometimes lie outright about what's going on.
In fact, after you win the argument and force it to let you into the Engine Room, the game initially refuses to tell you what's in there because it's sulking.
Unwinnable by Design: The player can render the game unwinnable easily and without warning. For instance, there are two entirely separate ways in which it is possible to get all the way to the end of the game and then learn that the game is now unwinnable because of something you didn't do right at the start.
One thing you need to do at the start, but probably won't think of if somebody doesn't warn you, is giving a dog a sandwich. Yes, giving a dog a sandwich. Apparently you can also have Ford do it when you revisit that event later, at least indirectly; if you buy a cheese sandwich as Ford and then give it to Arthur, he'll toss it to the dog as he runs back to his home.
There are about a dozen tools hidden throughout the game, two of which are in your soon-to-be-demolished house at the beginning (don't worry; if you take every item you see in the house before leaving, you'll have both). Near the very end of the game, Marvin will need just one of these tools to save you. Unless you use the psychic plant, the game will deliberately pick one that you don't have. (And you can't just bring them all because it's too crowded, and the floor is a grill that lets any dropped tools slip away.)