Fuck it, I'm drunk. Let's do this.
*The male pronoun is used here exclusively, because either A: I can't be bothered. or B: see above.
The singular most important skill for a Dungeon Master is learning.
A DM needs to absorb all information, images, and other sensory data that will inform his game.
What does that encompass?
A DM must convey an entire world to his players, one bit at at time. How much does a polar bear weigh? What are the prevailing winds in a given region? Does a witch float? All these things and infinitely more are vitally important to a given set of players in a given situation in a given game.
It is not, however, vitally important that a DM know exact details about these things, but he must have an idea.
The air-speed velocity of an unladen swallow is unlikely to ever come up in the course of regular gaming (if there is such a thing), but the DM should have an idea of how fast a bird can fly.
Especially if the players need to send an important message to a faraway kingdom where time is a factor.
Most important are visuals.
A DM provides all sensory information for the players, sight being the most important.
Humans (as well as elves and dwarves, for that matter) are visual creatures. The sense of sight is paramount to us. In general, the more a DM has seen, the more a player can see. This also ties back to general information. What do certain types of architecture look like? Furthermore, can players distinguish between a mortar joint and a dovetail? If not, a DM cannot just inform the players as to what type of joint is present, he must describe it visually, in the least.
Similarly, a DM must describe the effects of his world on all the other senses: smell, hearing, touch, and taste. Also included are any other senses specific to a fantasy setting, or otherwise not recognized by real world science, such as a players gut.
If a player gets "a bad feeling" from the obviously demonic artifact, a DM should know how that feels to a player. Does it twist their guts? Does it cause a sinking feeling? Perhaps vertigo? Additionally, characters who are sensitive to magic may feel their hairs raise when around powerful items or locations.
Always in the background of the DM's mind should be the idea that the players experience their world through him. Does the creature in front of them look like a giant, angry jello mold come to life, send chills down their spine, and smell like gym socks? DOES IT?
Fucking figure it out. It's your job. It is your challenge.
It is your art.