While a formal fallacy is an error in the structure of an argument, an informal fallacy tends to avoid the matter at hand entirely, demonstrating disregard for both your own position and that of those with whom you disagree. Both should be avoided, but online disagreements tend to degenerate into an exchange of the latter.
Ad Hominem
Instead of addressing the argument made, the response attacks the person making the argument. It is frustrating to receive this response, and if you respond to others in this fashion, it indicates weakness in your own position. If you are unable to support your argument, it may mean you are wrong, or it may mean your understanding is incomplete. In either case, do not call names, and recognize the difference between being told, "You're wrong because you're an idiot," and, "You're wrong, here's why, and by the way, you're an idiot, too."
Appeal to Popularity
It does not matter how many people agree with something. Belief does not create truth. Popular support does not make something right. "No one agrees with you" does not disprove anything. Progress is not achieved by following tradition, and discoveries are not made by following the crowd
Saying, "everyone knows Nazism is evil," does not prove that Nazism is evil, even though it is. An unpopular position is also not provel correct by virtue of its unpopularity. I have actually seen literal Nazis try to cite some variation of this as if it proved the validity of their philosophy. "Well, so-and-so wasn't accepted at first..." Riiiiiiiight...
Appeal to Authority
It does not matter who else agrees with any given position. Personalities and titles have no bearing on reason. There is some merit in citing an expert in a given field, but even experts can make mistakes, and there are experts in things that are wrong. I don't care how much you know about astrology or phrenology. I may accept your statements about what astrology or phrenology claims, but that is not proof of anything in the real world. Sorry.
False Equivalence
Comparing apples and oranges is very popular. So is comparing apples and orangutans. Things that are not equivalent should not be argued as though they were. It is sufficient to point out the flaw in the comparison. When someone argues that A is similar to B, examine the comparison made. Is there a distortion in scale? Are completely different things being compared based on an irrelevant shared characteristic?
"Republicans are literally Hitler," or, "Democrats are literally Stalin," are popular sentiments every election cycle. There may be some overlap in ideology, and some members of each party may truly espouse such authoritarianism, but adding the fallacy of composition to support a false equivalence makes an argument worse, not better.
False Dichotomy
When presented with option A or option B, why is there an implicit assumption that there are no alternatives beyond the ones presented? In politics, this is incredibly popular. "Submit to my policy, or people will die!" "Support my campaign or you support a murdering rapist who kicks puppies!"
Shifting the Burden of Proof
The one presenting the positive argument bears the burden of proof. When you ask someone to prove a negative, or say why a course of action should not be taken, you have avoided taking responsibility for defense of your own position.
This can get thorny when an intricate argument is being made, and sorting out who is making the positive claim that must be defended is not always so cut-and-dried. Keep discussion civil, and delve into the issue to see whose claim needs to be demonstrated.
The Traitorous Critic (Ergo Dcedo)
When an objection is raised, especially to political and economic matters, and the response is, "Well, why don't you move away," this is the fallacy at play.
I have seen many bumper stickers with the slogan, "If you can't stand behind the troops, feel free to stand in front of them." Is criticism of foreign policy really treason? Is it really rational to say that disagreement makes someone an enemy?
There may be a measure of merit in asking, "well, if you like the Canadian healthcare system so much more than that of the US, why don't you move to Canada?" However, their failure to expatriate is not proof that their argument is flawed. Likewise, objection to political overreach and government monopolization of services like healthcare is not rebutted by saying, "move to Somalia."
This only brushes the surface, but when a rebuttal causes you to cringe, it's potentially your gut reaction to someone avoiding the meat of the matter via an informal fallacy.
The truly difficult part is recognizing when you are about to fall into the trap of making such a fallacious argument yourself. We all do it from time to time. It's an easy trap. But knowing it is there in the first place is the first step toward avoiding it.
It's always important to examine what people are saying. One hears many arguments on a regular basis, designed to influence our thinking. We should be able to recognize any fallacy in those arguments. Thanks for this helpful list of informal fallacies.