Obviously, there are good behaviors such as being helpful, courteous, concise, and making sure the things you post are true and supported by evidence. But what should people avoid doing?
In approximate order of severity from least to most bad, these are the categories of misbehavior I've observed:
Spreading noise(the signal-to-noise ratio of the internet matters)
- Always posting about the same thing
- Sharing lower-quality versions of content than what you started with (please use or link to the original instead!)
- Frequently attempting to sate your insecurity by fishing for praise or attention
- Using 500 words when 50 would be enough (I have this bad habit; this article may be an example)
- Posting comments which are unrelated to the current discussion
- Posting text so incoherent that nobody can figure out what you were trying to say.
- Propagating self-replicating memes, depending on the meme
("I lost the game" and "the dress" and such...)
(as a general rule, don't spread self-replicating memes -- they are a social disease)
- Asking a human to answer questions which a computer or search engine could answer in 5 minutes or less
- Always taking, never giving anything back
- Asking a question then leaving before anyone can answer
- Asking if you can ask a question, especially if there will likely be a delay before an answer happens. How is anyone supposed to answer when you haven't actually asked the question? Pinging is okay, but only if you ping with data.
- Arguing from the bottom of the Hierarchy of Disagreement instead of trying to stay near the top. (resorting to insults and distractions and unsupported claims rather than using actual evidence to dispute the main point)
Breaking the internet's immune system
- Disabling comments on your posts when you fear the posts might point out that you're wrong
- Blocking communication in general (unless the argument has already escalated to the level of threats, as with GamerGate, or to block obvious trolls)
- Interfering with the use of self-correction facilities, such as Snopes or Wikipedia or comment moderation
Feeding the trolls
- Clicking on clickbait
- Sharing a syndicated version of content instead of the original
- Engaging a troll instead of ignoring/blocking them
- Playing games whose business model relies on peer pressure or spreading advertisements or infecting your friends to advance in the game
- In general, being an enabler for someone else's malignant schemes
- Sharing information without verifying it first
- Posting original content without doing the research first, a.k.a. creating new misinformation
- Posting satire without some type of explicit confirmation that it is satire (to subvert Poe's Law). (only a small, short note is needed though)
- Propagating self-replicating memes, if the meme has false info:
Forwarding chain mail, for example... ("Bill Gates will pay you to forward this email!" and such)
And virtually anything listed on Snopes.
- Writing / posting clickbait
- Posting at every opportunity just to share affiliate links
- In general, using or syndicating other people's content without attribution or proper linkage to the original, especially if you wrap it in a money-making scheme (such as advertisements for your personal profit) (for example, "upworthy" and similar sites, a.k.a. "freebooting")
- MLM/Pyramid schemes, selling false hope to the downtrodden (self-help materials, etc), and otherwise attempting to make money by exploiting flaws or loopholes instead of by creating actual things of value
Being a troll
- Posting crazy shit just to get a reaction for your own entertainment
- Being an ass online just because your real name isn't attached
- Saying or doing hateful things, in general
- Discriminating against (or making generalizations about) pretty much anyone based on what you think of their demographic rather than based on their personal individual actions -- or, condemning an entire population based only on the actions of a few jerks within that population.
- Supporting or increasing the oppression of an unprivileged demographic
- Causing drama
- Rage quitting
- Posting righteous outrage about things which don't actually affect you personally, or equating your feeling of being offended with someone else's real-life hardships. A.k.a. not checking your privilege at the door.
- Making a habit of posting divisive content instead of focusing on things which might actually bring people together or improve the world
Edit: Updated in 2020 to upgrade the severity of spreading misinformation from 3/7 to 5/7. It seemed like less of a problem back in 2015 when this article was created, but the years since then have revealed it to be a much bigger and more damaging sin than it originally appeared.