This answer follows my answer to a related question about types of non-deductive arguments.
The OP has three questions about informal logic.
- Are true premises and non-false conclusions enough or is there something more (or less)?
True premises and a false conclusion characterize an argument that is formally invalid. According to Douglas Walton, if one bases informal logic on Aristotle's dialectics, the form of the argument is not sufficient to evaluate it:
In informal logic, an argument is evaluated with respect to how it has been used in that particular case, within the framework of what is called a type of dialogue.
This would require identifying dialogue types and how they succeed or fail.
- Also, in the opposite direction, what would make one a bad argument in terms of informal logic then?
If informal logic is based on a framework of dialogue types then criteria used to evaluate an argument need to be considered separately for each dialogue type. Furthermore arguments may shift from one dialogue type to another. A fallacious or weak argument pattern in one dialogue type need not be fallacious in another. Walton provides an example using the ad hominem fallacy when a persuasion dialogue shifts to a quarrel. The evaluation of the argument needs to shift as well.
The ad baculum fallacy provides another example.
An argument that is reasonable in one type of dialogue can be fallacious in another type of conversational exchange. For example, the ad baculum argument (appeal to a threat or fear) is generally acceptable as a reasonable type of argument in negotiation dialogue.
However the evaluation of the use of ad baculum would not be acceptable in a critical dialogue.
...that would be very clearly regarded by everyone as an inappropriate and fallacious type of argument.
What would make one a bad argument depends on the dialogue type, dialogue shifts and the particular case.
- And why are sound arguments not enough?
Sound arguments are not enough because people engage in different dialogue types that in particular cases shift from one type to another. These arguments do not follow formal rules of validity and so need to be addressed on their own terms to effectively evaluate them.
Walton, D. New Methods for Evaluating Arguments. Inquiry: Critical Thinking Across the Disciplines. Summer. 1996. https://www.dougwalton.ca/papers%20in%20pdf/96NewMethod.pdf