First, none of it. I'm not getting paid for it and if it starts to feel like a job, I'll stop doing it. That's why, after my first campaign, I shelved my homebrew world. As much as I enjoy world building, the time I was spending on it was getting in the way of important responsibilities and other things I wanted to spend time on. It was also causing stress to keep ahead of the game.
So the most important "job" of the DM is to not treat it like a "job." Avoiding burnout is important, otherwise you'll hit that point where you won't want to DM anymore. Also, as
@Aldarc pointed out, don't let the game negatively impact your real-world responsibilities. Focus first on family and work. It will be hard to be a good DM if your life starts falling apart around you.
Most other points that I might bring up have been already touched upon in other posts, but I'll note that many suggestions given seem highly situational. For example:
Pacing. I find this one hard to parse. I agree with it for the most part. But I think it is an absolutely okay approach to let the players control the pacing. For example, if you and your players prefer a strongly sandbox-style campaign, the DM should feel compelled to "create some action." If you have a rich world with lots of hooks, the players can easily find action when they want it. This is why many (most?) players don't like more pure sandbox campaigns, but for those who do like this style, I think it works best when the DM doesn't force things to move along.
Antagonism. Again, I agree with this,
but... My last campaign was one that a ran in a manner that was antagonistic and would likely have put off many players. I was running a deadly megadungeon. I would show glee when players went down and disappointment when my big bads went down or whiffed a to-hit roll. My Discord and Foundry handle was "Hand of Orcus." An obituary list hung on the wall behind my DM chair (when I was running game in person). I think antagonistic DMing styles can be fun if the players buy into it. Obviously, you want everyone to have fun at the table. But DM versus the players style can be fun for some groups. I find that with this style of DMing fairness and hewing to the rules as written becomes more important. This leads to more rules lawyering than other styles of play, but despite the more commonly expressed feelings I read on the topic, I think rules lawyering can be fun.
Adaptability. I certainly think this is important. Then again this is something I feel I'm good at, so I'm sure that influences how important I feel this trait is. On the other hand, some DMs only want to run a specific system and only like to run a certain style of campaign. They may feel that they DM best when staying within their comfort zone, or they just found their game bliss and see no point in spending their free time running games that are less fun for them to run. Nothing wrong with this.