This is not a place for discussion of specific mechanics except as examples, please discuss things in general terms. This is not a place to argue that you are the only person who is right on a subject.
I think I favour more human, less ineffable, antagonists in general. ~Ivan
I like a wide variety of plot scales in a regular larp. Regular people and powers beyond the character's comprehension at either end of the spectrum for antagonists and NPCs, directional small personal plots to heroic stands to save existance as we know it on the plot scale. -Kelpie
Plot that flows from the state of the world and where PCs have agency to make it better or worse. Plot driven by understandable motivations, so you can hit it at multiple levels. Plot where PCs can and might fail and it is not the end of the world (Fire Mages in NFNC were the result of repeat PC failures on a short-term plot). Plot where you can throw yourself on one grenade, but not all of them at once as super-effective Heroic Sacrifices invalidate other PCs' actions. Plot that requires collaboration, not a certain PC power level (the old Purge X problem) --Jim
I think that a setting in which people do not feel they are obliged to do things which are often difficult to physrep, such as go about with full-body makeup, is preferable. I tend towards fantasy, but I guess sci-fi would also be cool. Definitely a world very different from ours though. ~Ivan
I also like a world that is not like this one(unless we get to break it.). For anything with a short arc - running a year or two - I'd like to see turmoil and world change(thus possibly less setting writing requirement), and to be right in the middle of it, and for there to be a spread of backgrounds so there's opportunity to play from any number of starting points for story/trope ideas for characters. -Kelpie
Be bold. Have a strong concept you can sell in a single sentence. Generic is not a selling point. We tried letting the PCs stake their claim on a canvas in NFNC and it was less effective. I think that for a short-run game, starting with a quite literal bang is the way forward, with the PCs picking up the pieces after The Fire Nation Attacks or whatever. Fantasy or Dune-Esque soft scifi space opera is plausible. Near-fuure scifi devolves into Camo N'Guns very fast and is less useful for our venues. Make it clear what the accepted morals are so PCs know when someone Crosses a Line. Avoid too many Designated Villains, make most antagonists about the actions, not the tools. --Jim
I like systems where your PC starts competent and new players feel useful and cool. Global hits, encounter-based so that you are encouraged to spend your cool stuff rather than save it for the inevitable boss. Calls should be uncommon so that most shouting is IC roleplay. Advancement is optional, and best when it is slow or broad. So rather than getting bigger numbers, you get new things or another use for a thing you already have. Nothing short of 0 Hits should render a PC inactive, FREEZE was horribly boring. Things being a bit overpowered is a safer failure mode than things being a waste of time, as it means all things will see uses. Modular, trait based systems are lovely as they are simple, flexible and promote archetypes. Alpha strikes are dull as they mean you win or they do nothing. --Jim
A system that is quick and easy to explain at the beginning of the year. -Joey
I very much like mask/makeup physreps that lie on a spectrum, like Empire Lineage. It allows people a wide choice of how much SFX to do, while restricting the number of "races" to a level at which you end up with plenty of each in play. Most people will go "human" so you want them to start easy to encourage variety--Jim
More self-reffable problems and time spent on prep to save time in uptime. We saved a lot of ref faff of "detect X on everything" by ruling that anything magic had a blue ribbon on it. Using OC or IC markers for "this is special" makes life so much easier. Same for drug lammies vs TT "is this a potion?" (possibly this is a System idea, I think it's more "physreps and admin" as it applies to any mechanic). --Jim
A setting (and system) that the people reffing are enthusiastic about, if the people running something like it then I think a lot of that can carry over. --Joey