Formal and informal fallacies are both persuasive forms of incorrect reasoning. Formal fallies are easy to identify because the structure of the formal language is what linguists called surface structure. For instance, a simple formal fallacy goes as such:
P1 A implies B
C Therefore A.
If we want to see a natural language version (glossing over some attacks on implications arising from intuition), it goes as:
P1 If Socrates is in the kitchen, then he is in the house.
P2 Socrates is in the house.
C Therefore, Socrates in the kitchen.
If one thinks through this circumstance, it's obvious that the reasoning is specious.
A formal fallacy is easy to identify because once translated into a formal language, the surface structure is mapped to a particular name. In the case of the argument:
the fallacy is known as affirming the consequent. Formal fallacies thus exist by essence of their logical form which can be identified syntactically as opposed to their semantics. Formalists love formal discourse because truths are determined relatively independent of meaning and thus are almost understandable of being free of normativty. Definitions suffer from near-universal consent (except from geniuses and cranks.) The ALU/CU of a typical machine can churn through these statements embarrassing humans in their proficiency.
Informal fallacies are also persuasive but logically incorrect, however, they rely on the deep structure of language. Searle calls it the Background. Computer scientists often just call it common sense. In essence, the dream of many linguistic formalists is to create an entire theory to describe these transformations. X-bar theory in the vein of Harris-Chomsky-Jackendoff is one example of how truth and meaning are interconnected through a series of formal grammars that attempt to explicate a natural grammar. Natural language processing and various schools of linguistics, including cognitive linguistics and psycholinguistics has greatly enriched the philosophy of language (though many logicians continue to work independently of the science).
So, to address your question:
What imparts informal fallacies with their fallacious nature?
This is a good question, and many who prefer formalisms sometimes struggle to understand the nature of the informal fallacy. I'll draw from Attacking Faulty Reasoning by Damer. From the chapter Introduction:
An informal fallacy is a misleading argument ("a group of statements, one more of which... the premises... support or provide evidence for another... the conclusion) whose statements violate one or more of three criteria:
3. Grounds for truth of conclusion
Well, you can see why formalists hate such definitions. They are highly normative. What is an acceptable proposition? How do you determine if the proposition is relevant? What does it mean to have adequate grounds for a conclusion? How do you tackle drawing inferences from natural language in which anaphor, metaphor, metonymy, synonymy and other figurative expressions play a heavy role in determining meaning? This isn't a small problem. This is the problem of grounding meaning in symbols, and is at the heart of why the Turing Test works to separate wheat from chaff, and plays prominently in the debate over the Chinese room.
TOULMIN'S USES OF ARGUMENT AND HITCHCOCK'S INFORMAL LOGIC 25 YEARS LATER
One philosopher who plays a prominent role in attacking formalism and advocating natural language philosophy is Stephen Toulmin who explored practical argumentation, not the toy problems of mathematicians, but the real-world kind that play a function in society. In his Uses of Argument he dispatches with the pedantry of logicians and looks to Anglo-American tradition of law for inspiration. He advocated a certain model with terminologies such as warrant, backing, rebuttal, qualifier, and domain-specific implying the importance of ontological pluralism and epistemological diversity in metaphysical presuppositions.
The ordinary language philosophers, such as Ryle, were/are big on stripping down language to common-sense positions and looking at praxis for determining truth. It might be understood as an empirical take on the philosophy of language setting aside the metaphysical speculation of logicians and embracing the self-evident truths of day-to-day life.
Now, to address the other part of your post. The thesis of the paper is that informal logic is not given it's due and that there are two major scandals involved. From page 6:
It is a scandal that the organizers of the World Congress of Philosophy can include sections on [various topics], but have not got around to including a section on the philosophy of argument.
This is important, because as a relatively upstart field informal logic is much better posed to address irrationalities such as "ideological fanaticism to culpable ignorance, and informal logic squarely faces this problem". Also:
... the teaching of informal logic in undergraduate philosophy courses in North America remains mired in antiquated theoretical conceptions, long disproved or at least questioned in the theoretical literature...
Is the criticism in the Wikipedia article enough to derail the subfields continued growth? Not according to the author of the paper who you present who makes a great case that after 50 years, there's been an explosion in interest and publication in the topic, and that further inroads can be made to strengthen the discipline such as addressing the two scandals listed.