What you should have known...

...Before Posting that comment

on the newest DD that you thought was "undeserving" of a DD.

Daily Deviations are a 24 hour feature. Nothing more.
GMs select DDs based on their own feelings towards a piece of art. As artists themselves, they do not always reflect every deviants beliefs and views of art¹.
A deviant receiving a DD does not have any say in the matter. In fact they probably didn't know they even had a chance at getting a DD!

...Before sending that GM a disrespectful note

about their newest given DD.

Gallery Moderators are volunteers
GMs are human just like you
GMs feature art they think is amazing. If they featured it they had a reason behind it.
Using Swear words and disrespect tends to get you ignored instead of an answer. Try writing a GM with respect.
Try seeing it their way from time to time. Perhaps the DD you dislike is actually amazing but you just haven't had a chance to look at it a certain way.

...Before writing that disrespectful note/comment/journal

about that new Daily Deviation.

It's against dA Policy and Rules to be disrespectful!²
It reflects badly on you. What you write will be seen by many other deviants and you will be viewed as a disrespectful deviant.
There is no need. If instead of spamming disrespect and hate, you simply wrote the GM Respectfully, it would all have been taken care of already!

...Before disrespecting that Gallery Moderator

GMs are Volunteers (Yes I said this already, it's something you need to remember)
They have taken hours and hours of their life in order to help this community
They were hand picked by moonbeam13 which means they can't of been horrible as they would have never been chosen.

...Before writing that dA Hate Article

that you recently posted or commented on supporting

Hate articles (or rant articles) don't help anyone.
Hate articles incite 'violence' and do not help the community improve.
Writing to the Help Desk, The Suggestions forum etc. is the correct thing to do.

...Before complaining

about anything.

You SIGNED UP for this website.
You can leave at any time.
You have to log in every day in order to complain
So you are already contradicting yourself by complaining.

No one enjoys listening to complaints.

...Before complaining that the dAr Category isn't art

and writing disrespectful comments to that effect

The deviantART Related Category is there to support the 2nd half of dA. It's Community.
In order for you to have written that disrespectful comment, you had to click on the deviation. Next time, just don't click on it and ignore it.

...Before having done anything like this

Consider what it might cause, weigh if that outcome is really worth it
If you were going to just Complain/Be Disrespectful etc., find out if there might be a better way to discuss the issue such as contacting the Admin or Volunteer who has concerned you or making a suggestion.

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References

¹ _{I have talked to deviants about all types of art including whether or not things like Comic Strips and Garfield (for example) are art. Some people say they are, others don't. This is the view I am speaking of.}

² _{See "Prohibited commentary is typically considered to be inherently disrespectful, aggressive, or otherwise abusive." Etiquette Policy as of January 15, 2011.}

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There are other ways to show dislike, sometimes the best way is to just ignore it. You don't ever have to click on a thumbnail you don't like.

Thanks to PurpelBlur for his feedback as well as suggestions for this article and proof reading prior to it being posted.