There are plenty of things out there that users add to posts that make them harder to read by adding useless information and text that doesn't add anything at all to the post. Some other things should be removed for alternate reasons as well, but keeping the post straight-forward and on-topic is the key goal.
Salutations, Signatures, and Taglines
There are plenty of different ways people add in extra little lines that more or less act as salutations and signatures. From basic things like saying "hello", "thanks in advance", and "you're welcome" to more subtle things like "hope this helps", "I found a solution", and "this may be late but..." in answers, these small phrases only add extra text that a user reads that may not be related to them.
As well, any kind of personal contact information should never be included in the body of a post.
Excess White-Space (Both Vertical and Horizontal)
Extra white space that is not needed extends the page and creates unnecessary scrollbars that make viewing and analyzing code much more difficult and inconvenient. A common example showing two versions of this is seen below:
$var1 = 1;
$var2 = 2;
$var3 = 3; // Now this line will cause a horizontal scrollbar with even fewer characters...
In this example, the extra line in between each line of code is useless. It only serves to add extra lines, while normally line breaks should separate blocks of code within a program that relate to each other. As well, the indentation of the last line makes no sense and creates a horizontal scrollbar. Re-indenting to match the rest of the code is appropriate. Be cautious when editing out white-space from posts, though, to make sure that the white-space you're removing is not relevant to the code. If unsure, ask the OP or leave it for someone else to deal with.
Comments, Comments, Comments
Keep comments in the comments. Don't reply to other users' comments by editing your response into your post. Only edit in relevant information that improves your question or answer. Editing in a notice asking why people are downvoting your question is never an appropriate thing to do.
Offers of Financial Reward
This is not the place for hiring someone to do work for you, even for the simplest of tasks. You are not allowed to offer any sort of financial compensation for answering any questions here. If you need someone to build an entire website or do some other form of contract work for you, try posting an ad on Careers. Usually the question is still salvageable by just removing the references of the reward, so pay close attention to not remove vital parts.
Degradation of Self or Others
Most often this occurs with users calling themselves "noobs". Everyone has to start somewhere, we were all "noobs" at one time, and it doesn't belong in the question. Of course, calling someone else a noob (or equivalent) is not appropriate either. Similarly, using things like "I'm an idiot" aren't great.
Continuing of degradation, the FAQ clearly states you should be nice and treats others with respect. The use of words such as f*** and b**** (yes, I censored them) are not appropriate and definitely would not be considered "being nice" by anyone. Even calling someone "stupid" can be considered offensive and can result in disciplinary action.
Apologizing for asking a question is a sure-fire way for people to not take it as seriously. If there's a duplicate question out there or you asked in the wrong place, it will get closed as a duplicate or people will help you find the right place to ask. If you're not a native English speaker, try your best. Eventually someone will see your question and care enough to fix it up so that it is readable to anyone.
Religion or Other Beliefs
If you're not answering a question specifically about religion on a religion-related Stack Exchange site, chances are it has no place in your content. Religion has caused wars in the real world and we're not here to start any of those. If the religious content is not directly related to the question at hand, it should be left out. This also means not using religion as examples for a question. For example, if you were building a function for a religious website, you should change your code to remove any mentions of religion in the function in order to avoid conflict here. Remember that your problem should be helpful and useful to anyone who might have a similar problem in the future.