Animals Hate You: At the center of The Maze, unlocking a cage releases a hawk, a snake, and a cat. You're then given a choice as to what to do with them. Three out of four of your options lead to getting the crap mauled out of you.
Angry Guard Dog: Two of em, guarding the church. Very, very mean dogs.
Awesome, but Impractical: There's a large, wicked looking battleaxe that you can pick up very early on. Too bad the only thing it's good for is killing YOURSELF (Justified somewhat, as the house is full of ghosts). You'll later use it to break open a particularly stubborn cookie jar. And the axe breaks in the process.
Bottomless Pits: You need to deposit the Big Bad into one of these to be rid of him. And quickly, or he'll be the one to do the depositing.
Chekhov's Gun: The cross (chalice in the NES version).
Conservation of Ninjutsu: A strange example: You can defeat a single zombie by using the power of the pendant, but the pendant won't work on a whole horde of zombies. At the same time, you can't escape the lone zombies, yet you can escape the zombie hordes by simply ignoring them. Of course, newer players wouldn't know to do this.
The Corruption: What happens to you if you don't get rid of that damn ruby.
That's on the NES version. In all other versions, just entering the mansion subjects you with it, so you better act REALLY quick or you die. Fortunately, the mansion itself doesn't kill you nearly as quick as the ruby did in the NES version.
Distressed Damsel: Your main reason for being in the hellhouse is to locate your sister, and prevent her from becoming a Dead Little Sister.
Only in the NES versions. In all other versions it's your little brother that's missing.
Drowning Pit: The bathroom quickly turns into one, if you're silly enough to turn on the sink. Though this turns out to be the only way to get into the final room.
Every Car Is a Pinto: The very first thing you need to do in the game is get the hell out of your wrecked car. You have three turns or so, before it goes boom.
False Reassurance: "Thank you for coming back for me, my love. You will be mine forever..." She's dead, and you soon will be if you see this line...
Forbidden Fruit: The greenhouse contains the most literal example.
Gender Flip: The original MacVenture version had you braving the house to rescue your younger brother.
A quite likely explanation is that, since the NES port was originally Japanese, they figured saving an older sister would appeal more to the gaming audience at the time.
Getting Crap Past the Radar: Unlike Shadowgate, this game likes SHOWING you just what it is that's biting and gnawing and ripping your flesh off.
Although there were things that were toned down. I remember a preproduction screenshot in Nintendo Power that showed a ghost that carried his head in one hand (the one that appears in the attic cell). In the actual game his head is on top of his shoulders.
He's carrying his head in most other versions.
Ghost Butler: The front door, of course, refuses to let you out once you're inside. There's another one upstairs, that proves a little more fatal if you're silly enough to use it.
Giant Space Flea from Nowhere: The red tomato-monster thing that you need to uncage the hawk for. Even the narrative text gives up trying to make sense of the event.
Giant Spider: Amusingly enough (and in perhaps the only example in the entire MacVenture series), the game warns you multiple times that leaping into the den of the resident Giant Spider will probably be a bad idea. Ignoring the warnings leads to your being eaten, prefaced by the humorously patronizing message "Well, what do you know; it's a giant spider."
What the hell keeps killing you for no apparent reason (the ruby).
If you examine the ruby, you'll get a pretty good hint that it's something you don't want. Also, this is only in the NES game. In all other versions, this happens whether you have the ruby (a star in all other versions) or not. Fortunately it takes much, MUCH longer in other versions, essentially giving the whole game a time limit.
Rouge Angles of Satin: "This phantasm must have come from the valley of death. It stares at you malisciously."
Schmuck Bait: Two of them: the prison cell at the top of the ladder, and the giant spider in the tunnel. A lesser known one is that you can crawl inside the coffin in the maze (often accidentally by going east) and you die.
Sealed Evil in a Can: Dracan is, fortunately, buried within several tons of ice. Unfortunately, you need to get him out of there in order to actually kill him.
Shout Out: You can find the graves of both Ace Harding from Déjà Vu, and of Talimar, the Big Bad of Shadowgate inside of the maze. Also, the phonograph in the house proper plays a (horribly off-key) tune from Shadowgate.