Door of Doom

This must be where they keep the kitchenware.

There's a door ahead. Based on its appearance or its context — usually standing there with no actual wall around it — you just know something amazing and dangerous lies on the other side. The Door of Doom isn't just a physical trope of a door being there, but a narrative device. Because the door is so obviously doomful, the characters recognize that going through is a big decision.

Compare Cool Gate. Not to be confused with Doom Doors.

Examples of Door of Doom include:

Anime and Manga

The Gate of Truth in Fullmetal Alchemist.

Film

The door to the maze in Labyrinth.
Jurassic Park: John Hammond deliberately invokes this by having the guided tour of the island begin by entering a large, impressive door, but it's more for show than anything.

Ian Malcolm: What do they keep back there, King Kong?

The entrance to Toontown in Who Framed Roger Rabbit?.
King Kong: The huge door in the wall surrounding the interior of the island.
We get about five minutes worth of this in Andrei Tarkovsky's Stalker, in which the main characters stare at an omnious door, contemplating whether or not they should go in.

Literature

The archway in the Ministry of Magic in the fifth Harry Potter.
Narnia has a number of these, including one near the end of Prince Caspian. Played with in The Last Battle - the 'door of doom' is just the door of a stable....
The three Doors in The Drawing of the Three, the second book of Stephen King's The Dark Tower epic.
In House of Leaves, the door that appears in Navidson's house after the family comes back from vacation, which wasn't there before.
The door in Dante's Inferno, with its famous (and much referenced and parodied) inscription "Abandon all hope, ye who enter here."
- Parodied in Eric, where the inscription has been crossed out in favor of "You Don't Have To Be Damned To Work Here, But It Helps!!!"
The Doors of Durin (west gate of Moria) in The Lord of the Rings.
- As is the Dimholdt on the Paths of the Dead, especially in the film version.

"The way is shut. It was made by those who are Dead, and the Dead keep it. The way is shut."

Subverted in The Princess Bride (the book), where the actual danger lies on the door itself.
The entrance to the Temple of Doom in Temple was the Incan version: a giant rock blocking a passageway, with very detailed pictures of people dying all around it.

Video Games

Possibly the final door leading to Tabuu in Super Smash Bros. Brawl, being a gigantic glowing door leading to the final boss that requires you to defeat every opponent you've ever faced to pass through it.
Both played straight and subverted in the Kingdom Hearts series.
The door that leads to the first boss in Silent Hill 3.
The red boss door in Yoshi's Island. Bonus for being twice the size for the Final Boss.
- Super Mario World did it first: Haunted Houses, Castles, and Boss Rooms all have some sort of doomy door.
- Not to mention the Thousand-Year Door. The earth all around it cracks when it opens!
The door leading to the Fugue plane in Neverwinter Nights 2: Mask of the Betrayer.
The Oblivion Gates in The Elder Scrolls 4 are big burning arches that scorch the area around them, turn the sky above it red and ominous, and the plants from Oblivion also grow around it too.
- Specifically, what is on the other side leaks over. Which is a barren wasteland of blackened islands in a sea of lava.
The Dimensional Gateways in the Warcraft setting are these. The prime example is the Dark Portal that leads to Outland, a world that had so many of these gates opened that they tore the world apart and sent what was left into the Twisting Nether.
God of War has three main flavours of door: large, very very large, and "yeah, definitely a boss here" large. (Or, from a god's point of view, miniature, small, and medium).
In Minecraft both the Nether and End Portals. They don't look great like many other things on the list, but considering how the rest of the game looks, they're pretty hellish. Some people have even made their portal rooms the definition of this trope. Just look here.

Web Animation

The third Charlie the Unicorn flash has one of these. Er...sort of. It turns out that the door is not as fantastic and mysterious as advertised.

Western Animation

Futurama has a Show Within a Show that uses this: The Scary Door.
In The Real Ghostbusters episode "Knock, Knock", a literal Door of Doom is featured.

"Do not open until doomsday!"

Other Media

The painting That Which I Should Have Done I Did Not Do (also called The Door) is one of these. Subverted in that it represents a wasted life - not going through it was the wrong choice.