Play-by-post games (hereafter referred to as PBP) are online text-based role-playing games in which players interact with one another and a predefined environment via text. It is a subset of the online role-playing community which caters to both gamers and creative writers.
As a result of the medium, PBP games tend to attract the creative and imaginative among us. If this sounds repetitious considering that roleplaying game enthusiasts, in general, tend to fit this description, you're not wrong. What sets these players apart then? First, perhaps, it would useful to identify the characteristics they share in common.
RPG enthusiasts tend to be imaginative. That is, they don't need anything beyond their imaginations to envision amazing lands, terrible villains, and terrific tales; they are are not bound by the physical limitations of their console or computer's graphics card.
RPG enthusiasts tend to be creative. That is, they apply their skills, imaginations, and abilities to the realization of the products of their imaginations. There are countless examples of this creative energy made manifest. Cosplay (costume play) is one such example wherein enthusiasts put their creative energy into the production of costumes which demonstrate not only their skill at the craft, but also their passion for the subject of their work. Their creative efforts can also be seen in the various accoutrement associated with the game they enjoy: character portraits, emotionally compelling background stories, customized character sheets, custom-made dice, etc. Suffice it to say that someone likely told them at some point to "get a hobby" and they made their hobby their passion.
RPG enthusiasts tend to be social outcasts. Popular culture being extremely conformist, shallow, and generally lacking in creative imagination, RPG enthusiasts are still social beings, and so find or create their own communities in which they can obtain a sense of belonging. These communities based on areas of specific interest are often called "fandoms," and they are a growing influence on popular culture thanks in part to the creative efforts of people online. A good video overview and treatment of fandom's future here.
PBP gamers are a subset of RPG enthusiasts, so they share all the common characteristics of an RPG enthusiast, the previous characteristics of which are by no means the totality. What sets the PBP gamer apart from the more general RPG enthusiast is the circumstances of his life which affect his choices, the medium of choice for his creative efforts, and his preference of the written word over the visual simulacrum of the products of his imagination.
PBP gamers tend to either live too distantly from other gamers to make regular gatherings practical, or else have busy lives which make scheduling such multi-hour gatherings nigh impossible. In those rare instances where these are not factors, there are also the factors of agreed upon meeting places (the FLGS being a dying breed), the sometimes questionable hygiene and housekeeping preferences of other gamers, and the sometimes atrophied social skills of their fellow gamers.
In short, PBP gamers usually lack the time and opportunities to directly engage their fellow gamers in person, and often have preferences which disincentivize such. This influences their choice of medium for the creative outlet of their imaginations. The internet becomes the only means by which to satisfy their desire to play roleplaying games in a manner familiar to them. It is at this point that these gamers start doing research and find the following options:
Skype chat, IM services, chat room-based games, and other similar real-time, online means of bringing gamers together remotely for a gaming session. While these platforms offer a variety of tools and varying levels of quality in their experiences, and satisfy the problem of geographic distance, the real-time aspect of the game excludes those who have busy lives and/or unpredictable schedules. These type of games do not usually last very long as a result, though, of course, there are exception.
Play-by-email games use e-mail as a means of mass correspondence, but the time delay involved in such games often rules out much in the way of personal attachment to a continuing narrative.
Play-by-post games address the issue of geographic distance through an online meeting place just as chat-based games do, but also address the problem that adults face in finding time for their favorite hobby. Unlike play-by-email games, however, quality PBP games have posting requirements confined enough to keep the game interesting and active, but broad enough to allow for a great diversity in people's schedules and busy lives to be accommodated.
The following factors do not affect PBP games:
Living in proximity to other gamers
A local FLGS to play neutral host
The personal habits of other
Therefore, what makes a successful PBP gamer therefore is not any of those things above, but a few, simple character traits that are personal in nature.
You own yourself. No one else controls your body or your thoughts, so you own yourself. As a result, you own the consequences of your actions. You alone are responsible for meeting any obligations to which you have voluntarily agreed. PBP depends on players to be responsible. Those who repeatedly fail to take such responsibility invariably end up becoming "flakes"--players who just stop posting, usually without even providing notice of their intent to do so.
Playing the part of a character in a PBP story/adventure/campaign requires that the player read all new posts in the thread in which his character is involved, and then to respond to his fellow player characters and the changing environment and circumstances in which they are involved. Done well, and by a player comfortable with writing, this can take as little as half an hour, to upwards of an hour. So a player must be able to manage his own schedule and allocate an hour or two each week to keeping up with the adventure and discussion threads in which he's involved.
Playing the role of a GM requires a vastly larger commitment of involvement, time, and effort.
The ability to effectively communicate through text should be an obvious necessity, and cannot be oversold. As a player, one needs to be able to manipulate text sufficiently that one's intended actions, thoughts, and dialogic interactions with others in the environment are readily understood by one's fellow players and the GM. As a GM, this ability becomes even more important as the GM is not controlling a single player, but the environments surrounding each of the players for which he is responsible for crafting a narrative in which they are all involved. This includes the ability to describe the mood, physical environment, personalities, expressions, dialog, tones of voice, everything a player character's sensory abilities can receive as input. A GM must, therefore, be not only a capable writer, but a good writer.