Title change suggested by @lah7; Thanks for the tip!
Preface
Figured I would write this up real quick as I noticed some users were being long-winded and making posts so long that they might be tl;dr'd. To prevent this, you should learn a little-known secret about Discourse.
Despite the best efforts of the Discourse team, it still succumbs to a trap of other forum software; it is not optimized for formal, proper writing. Also, because of people's short attention spans (which have only become worse with time) you can't just bang out extra-long paragraphs and expect them to be read. So here are some basic tips for writing in Discourse I found tend to work well for me. If your style differs, it isn't wrong, though you could probably optimize it by applying some of the techniques shown below:
Avoid proper paragraph-style writing
If it's more than six lines, people tend to skip long swaths of text. Even that bit in the preface was stretching it, if there are enough rows of text on the screen, one can lose concentration, skip lines and lose interest in your own post.
There are a few ways of resolving this, but there is no one way better I have found than to avoid writing proper paragraphs if they'll be more than seven lines. Terrible as that sounds, Discourse is a place for informal writing and be informally informative - a sort of "Cliff notes", off-the-cuff and bite-sized morsels of information for people who are seeking a solution to a problem, or to just chat about current trends.
Avoid widows
In the context of writing, a Widow is a single word at the beginning of a new line, that is located at the end of a sentence. Highly conditional and variable depending on element width, widows can make a block of text less appealing. So it may be of benefit to restructure a block of text so it doesn't end on a single word in a new line and deter picky readers.
Conversely, you can use widows to keep the reader's attention. Ending a sentence with a single word on a new line can lead into your next point on the same line; it can exploit a nature of reading by replacing that feeling of emptiness with more information; something I unintentionally do often.
Break it up with markdown
Add emphasis where it absolutely needs to be. Define elements of interest with bold and italic formatting. Try to use fixed-width text for program names if writing about a piece of software or providing a command. As the moderating team has shown me, you can also use the <kbd> tag for keypresses if you are writing about a sequence of keys, i.e. (Fn +) Alt + F2.
There are a variety of ways to achieve this. Try it! (You can also use some HTML tags!)
Do this
Segmenting the text; splitting it up into multiple parts with headings and subheadings using hashtags (with a space after) can dramatically improve the readability of a long post which can better convey your point of interest. In doing this, you can refer back to earlier sections using headings so the reader can look over previous text for better understanding of subsequent sections.
Use superscript for extra information
There are a variety of ways to handle this; one could use unicode superscript numerals for footnotes, or just place a bit of information below an informative text, but the most simple way I found to draw attention toward extra text is to use superscript. Using the HTML <sup> tag also opens up more creative use of emojis, which can spice up a post without looking too obnoxious
<sub>is supported for subscript also!
Addendum: Image small print
If you are sharing a picture, a brief context about the image is nice, but there is one problem with that; if you keep the text normal-size you may find it difficult to cleanly flow into the next piece of information because the text below the image is the focus of attention, not necessarily the content after it.
To resolve this, you can use superscript on a new line below the image. This is great for citing an image source, or presenting the context or ancillary details of an image, and it also spaces out nicely so the details are separate from the main body, despite being inside of it.
Use blockquote for extra information
Alternative approaches, caveats, ancillary information only tangentially related; all of this shouldn't be in your main point unless it absolutely does not fit being in a blockquote. To make a quote, begin a new line with the greater-than symbol (>). That's it. And that may be the difference between beneficial "Margin text" being read or being ignored.
Use as few words as possible
...But not to where you appear like a robot void of personality. Limiting your writing can help to not only get a point across quicker, but also show that you value the time of the reader. You may have spent hours on something that can be read in only fifteen minutes, but people will like you more if you simply get to the point, unless there is some beneficial information or warning as a preface.