What I learned so far about writing a quest is this:
(Keep in mind that I that I got most of this from reading quests, not writing them. I do write one, but its not very long yet. So take my advice with a grain of salt.)
Some of this has been said already, but its worth repeating.
- Writing is more fun and more work than I expected.
As others have said, writing takes time, and going over everything
again for mistakes after you already worked on it for a while and read it over and over to get the style right is pretty exhausting for me. Sometimes I just skipped the proofreading, and looking over those parts is cringeworthy.
That being said, I have a lot of fun writing, since I put some work into preparing the quest (from mechanics to worldbuilding) and I finally get to show people.
- Its hard to get a decent balance between wordbuilding/exposition and plot.
In a quest players are supposed to make informed decisions. That means they need to know all the relevant things about the decision. Getting all that information across without it getting boring is
hard. Maybe start with some smaller choices before moving on to the bigger ones when you have had time to worldbuild a little. Idk, I failed at this imho.
- Don't be disappointed when few people participate. We are a small forum, and if its your first story people will be hesitant to invest time into reading it. Especially if its a fandom that isn't well known or god forbid, original. Soldier on. People will come eventually.
- Choose the story you want to tell with the quest format in mind. Some formats do well with some types of story others with others. Is your quest turn based or narrative based? Are you rolling die or do you decide what happens depending on what furthers the plot? Are you going to have plan votes or are you tallying by line. Will you perhaps combine votes to reflect as many of the voting base's ideas as possible? Imho, knowing that is important.
- Also, as long as you credit people and you aren't directly copying them I see no harm in using other's ideas. Take a quest you like and move it to a different setting for example. That way, not only will you know what you can expect, you can show others what to expect just by pointing out the work you are basing it on because....
- It really helps if your readers and you are on the same page.
I suspect this is true for regular stories as well, but its even more important in a quest. Make sure your readers have enough information to make proper decisions. Nothing is worse for them than getting blindsided by something that they should have seen coming IC.
You as the author will always know a lot more about the world you are writing than any three readers put together. Its important to clearly lay out all the relevant information. Unfortunately, that's not too easy without getting boring, and plays into the whole exposition thing I mentioned above.
But it also helps if players know what kind of story you want to tell. If they know you are writing crack they will vote differently than if you are writing a hardcore number crunch quest.
- Your enjoyment in the quest is the most important thing.
The quest will end as soon as you stop enjoying to write it. Others may quit if they are unhappy with a turn of events and the quest can still go on, but if you quit its game over. I've seen too many great quests end, just because readers insisted on doing something the qm wasn't happy with.
So don't be afraid to veto, change or even retcon whatever you want. As long as you don't abuse this people will understand. A simple "I don't feel comfortable writing this" is all you need and anybody who has a problem with that can go write their own goddam quest. /rant ;-)
-And finally: Just do it.
If you have a decent idea you can see yourself pulling off, just start the quest. You will never be able to prepare perfectly before you start and its better to just get going. That's how you get better after all.
As soon as your idea is fleshed out enough, start writing. You can rework it during the process if you run into any snags.
As for what I particularly want to know about, I guess the most confusing thing to me is, how do you decide what should happen next, especially fast enough to have a good update speed. Votes are going to determine some of what happens, but the QM still has to invent both the voting options and the details of the winning option.
That's a really hard question to answer for me and I think it heavily depends on the type of quest you want to run.
Most of what I could say to this has already been said, but I'll echo this statement: The more equal your vote options are, the better. If all your options have pros and cons and none are objectively better than the others, then the discussion really get going.
Also, do not offer options you are not prepared to write. Because eventually one of those will get voted in.
Also also, consequences don't have to be instant. You can have them pop up a few updates later.
There isn't one specific quest that I want to run though. Half the fun of co-writing is trading ideas with someone else and developing a concept together, right?

I have in the past posted some quest ideas, and I could collect those up to show to someone interested it co-writing. But, it seems like if one person proposes a concept then they are expected to be the leader or senior partner of the collaboration, which I really don't want to do. I feel like I need the educational experience of being a junior partner first.
You can pm me those ideas if you want. I have some ideas as well and I always have fun bouncing them off people.
Anyway, I'm looking for something fun to write during the voting periods of my other quest, so perhaps we can find something that works.
I'm just starting out as well, so we can be noobs together XD