What is City of Mist?
In three words: Queer. Supernatural. Batman.
As for the long version:
First, I'll advise looking at the
Quick Start PDF, freely available on the official
City of Mist website. I just won't be doing a better job at explaining the setting, system, philosophy of CoM than that one introductory document.
At most can I try to rephrase some of it with my own words:
It's a game about normal humans living in a modern city in this day and age. Normal humans that also happen to be vessels for powers far greater than themselves, modern avatars of concepts, stories, ideals that have existed since the dawn of humankind.
The emphasis however is not so much on the powers themselves than on how they define, shape, change their owner, with a simple yet very relevant system that encourages using them alongside more mundane qualities in original ways.
For example, while it glosses over questions like how many fireballs a journalist channeling primordial fire can throw per minute, it provides a built-in framework for them to use their pyrokinesis to set up the perfect lighting for that one picture they need for their next article.
Also, there's a more specific question I can actually answer: Why I think CoM is a good fit for the PBP LARP format.
It's character driven first and foremost.
A character sheet is made of sixteen story hooks and not a single figure. You don't actually have to worry about power level, party roles (like healer, tank, DPS...), combat efficiency and whatnot.
What the system mechanically encourages however is to have a rich and complex character, with a lot of evocative elements that can easily be summoned during scenes. As long as it inflames your imagination,
amateur gardener is a better asset than a
laser sword you have no idea what to do about.
For people from a L5R (or GURPS) background, think of a character made solely of fluff advantages and disadvantages yet still "mechanically competent".
Actions are naturally descriptive, cinematic, verbose, and thus offer a great base to write good posts.
An example: If you wish to have your PC cross a river, in a lot of TTRPG, it would be, out of the box, a relatively bland Athletics roll. In CoM, it's an opportunity to delve deeper into your PC's
medal-winning triathlete past,
parkour artist present, or
sea-parter mythos.
Of course, you may object that you can add flavor and color on your own to even something like a basic Athletics roll. This is very true. But it's not something that the system
encourages or
supports you doing. Meanwhile, CoM, through its mechanisms, gives you the thumb up for being more narrative about your actions and provides a well of leads about how to do just that.