If you are replying to someone, try to always quote their post (while removing unnecessary parts of the post, like advised in the OP). Also try not to modify the header of quote (the the date and message link). The reason for this is that some people are using quote notifying services, and they would miss your reply if you just write a post beneath the person you are responding to. Also someone else might post while you were typing, so it wouldn't be clear to whom you are addressing your post.
This doesn't happen too often, but still it annoys me when I see people replying to my posts without quoting me.

It's true, especially if threads have lots of replies. Simply posting will make your post looks a bit off-context, and readers have to guess who you want to discuss with, OP or anyone else posted previously.
Making posts on mobile is one of worst experience, for quoting issue you mentioned, and it takes more time too.
If you don't want to edit quote, I think you can quickly quote like this:
This quote style only let users know they are mentioned in your posts, but it does not giving them more details on which of their posts you want to discuss.

Hence, if I want to let others know which specific posts they want to discuss with, I will do have to quickly quote like this:
Then, replying. In this case, I think it is better to spend a few seconds to copy the whole quote details (post author, topic number, message number, date of post).
I don't think my thread is helpful for all, because there are lots of users don't care about those things even they see my thread. They might fastly ignore my thread at first glance, because it contains so many words inside. Only serious users will do care to spend time and read it.
In some good payment campaigns, posts in Spam mega threads won't be counted. The definition and identification of Spam Megathreads depend on perspective of each manager.

One more important thing to add, don't simply leave negative reputation(s) unless you don't mind receiving them in return as a form of revenge, unless you're a scam victim or the recipient really deserved it.

I think using ~snip~ when quoting a post is a very good idea if the quote is a wall of text or very long and if you just want to highlight one part of it and reply to it rather than the whole post.

You forgot to add that users will Tut-Tut you for using the rare profanity and then cheer on or lament TMAN's absence for his copious amounts of swearing and abuse.

#DoubleStandards.

It is not only those off-context posts that I'm getting uncomfortable with, but also those cliche statements that they are saying over and over again even if there's a statement like that already. Same thoughts and same ideas are got me annoyed, and that only means that they are not reading the whole discussion because they still reply with the same concepts.

You don't need ro quote if you are making a general reply to the OP.
You don't need to quote if you are replying to a post that is immediately above your reply.
You don't need to quote images.

Oh! I'm guilty of this one but not totally . For me, I used to quote the post above mine to avoid confusion. To show that I'm reacting to the statement of that particular member not on OP's.

But unlike the others, I do not quote the whole post especially when it is long. I only pick the sentence or phrase I want to emphasize (like what I did right now). But if there's an instance that the thought will lose once you cut some of its parts then I simply use [snip] or [...] so that the thread will not flood.

I guess my way still had a right etiquette. What do you think?

Or just put up the dots like this!

It's even best if in some instances we give references to another thread where the answers has already been discussed for that matter.

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Just a thought . . .
There should have been auto-link generation when we put @userA within the post. Like those in another social media platforms. It could have been easy to click the user profile.

There are some quote patterns: multiple quotes; nested/ pyramid quotes.

What you mentioned is fine, nothing wrong with the style but you can improve by snip unnecessary parts of each quote to keep each quote shorter and points straight to what you want to discuss with.

To help readers understand who is the user you want to discuss with, use the horizontal line with hr to seperate between each user.

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The below quote style is called as Nested/ pyramid quotes. This is a very annoying style.
If I comment with such quote, readers don't know who I am discussing with or which post of yours I want to discuss with.

You can avoid it easily with a few spent seconds but if you want to have a tool, here it is User Script: Automatically remove nested quotes v1.1.1