Note: I'm basing my statements solely off of the experience I've had on b12, which has almost completely consisted of Arms Races. I'm also referring to the reader, not the OP, when I say "you". Lastly, it may seem like I'm promoting my own games here, and I am. Of course I am. Come on now. Nothing like a little advertisement to draw the crowds.
Everything begins, in my opinion, with the GM. People won't stick around for someone that doesn't have gravitas, and a shitty GM will just piss people off. Not everyone can weave a compelling tale, so it's best to find a writing style you're good at and a genre you're comfortable with and work from there. Fantasy, Sci-Fi, Medieval, and Modern settings, just to name a few, can meld with a wide variety of different writing styles. I had "hero" characters in my first Arms Race, Industrialized Warfare, but as time went on I realized the scope of each battle surpassed what any one character would experience, and so when the war switched from minor skirmishes to all-out war my writing style adapted to reflect that. A short-lived game I ran, Blood of Champions, was heavily focused on individuals, and so the style IWAR started with fit much better.
Setting is also very important, though with a skilled GM you could have a game about working in a fast food joint wind up entertaining. When I start any game the first thing I look at is setting saturation. IWAR was set in the WWI-era, but with a science fiction twist, which separated it from the couple of other games set in the same era. There were no gladiatorial arms races when I started work on BoC. My current games, WarBiz and SCP: Containment Race are both based on popular settings among forumgoers here (PlanetSide 2 and SCP Foundation, respectively). While both settings are popular, nobody was currently utilizing them on the forum and so both have not only drawn healthy populations, but they've also pulled in Arms Race newbies. It helps that apparently people liked IWAR and the one and a half turns of BoC, but that just circles us back to the GM part of the equation.
Mechanically I personally try to include some new twist to keep things interesting. People here like to break things so when you give them something new to toy with it can keep them intrigued for a little bit. I roll 2d6 to determine outcomes of design and revision actions in my ARs with 6, 7, and 8 all counting as an Average result. Other games use 1d6, 1d4+1d3-1, 2d4, or even
a deck. By doing something different people didn't have preconceived ideas about how it'd work. Every Arms Race past IWAR has included some other major shift in mechanics - BoC was a total overhaul of everything, WarBiz utilizes defined Class Loadouts, and SCP: CR, well, that's still early enough that I'm not gonna spill those beans yet
A lot of what our Hiatus Lord and Savior Nuke said is spot on. Especially when it comes to freedom for the players. Nobody want's to be on-rails for their whole experience. Even the
illusion of choice is better than nothing (I'm looking at you, Telltale Games, RIP). One of my favorite things to say to my
victims players came from watching Critical Role (a D&D series with a bunch of nerdy-ass voice actors) - "You can certainly try." It's best to have loose parameters and not a strict guideline when it comes to dealing with the infinite capability of a group of human minds to utterly break something.
In regard to rules, a GM should never be afraid to change something up. I had to expand the range of modifiers with my 2d6 system after realizing overly ambitious actions weren't quite punishable enough without going overboard. So long as you are clear and concise about your rule changes there should be minimal issue.
Most importantly, know how to write. This one may seem obvious, but to put it bluntly some people have no right to put written words together. I'm not saying these people are dumb or anything, there are a hundred different reasons for why someone's not that great when working with the English language. It's not everyone's forte, either. But even more importantly than that, you need to
enjoy writing. Some of my IWAR updates took 4-6 hours of writing, and I'm the kind of asshole that tries to knock things out in a single session. If I didn't enjoy writing I'd have probably brained myself.
Research is also very, very important. Know your setting. I specifically keep naval stuff out of my games because I have
very limited knowledge on the subject and don't feel like dedicating the bazillion hours I'd need to feel confident in my ability to emulate it properly. Likewise, if you're running a game like one of them FEF jawns you should probably have an idea on how Fire Emblem works.
Finally, do drugs and drink alcohol. It totally doesn't impact writing at all.