Mariner – although what you wrote is correct, that rule does not apply to the sentence in question.
“I recently joined this forum, and I am wondering why it is that most of the people that post here cannot put together a proper sentence.”
Contending that the coordinating conjunction is adequate separation, some writers will leave out the comma in a sentence with short, balanced independent clauses. If there is ever any doubt, however, use the comma, as it is ALWAYS correct in this situation. So, while the comma does belong there, it’s not because it’s in a series of three or more items, but because each line on either side of the comma can stand on its own as a full sentence.
And Zishe, while “The comma after “atrocious” may be debatable”, it certainly is not “dead wrong”.