What makes a paper publishable?
In my area, software engineering, the criteria for having a paper accepted (which implies later publication) are usually listed in the call for papers of the publication venue (conference or journal). They typically include the following:


Novelty. The paper should contribute to the knowledge in its field. This could either include a new problem formulation, and a new idea to approach an existing problem.
Relevance. The paper should make a point in why the presented work is relevant. 
Soundness. The work presented in the paper should apply a sound methodology. The argumentation presented in the paper (in particular, technical reasoning) should withstand scrutiny and be reproducible by experts in the field. 
Presentation Quality. The paper should be logically organized, self-contained, use appropriate scientific language and writing style, and be free of major language flaws. In some research directions, there is a standard paper organization that should be followed (for example: Introduction, Related Work, Methodology, Results, Discussion, Conclusion).
Appropriate consideration of related work. The paper needs to depict its own contribution within the landscape of existing research. Closely related work has to be referenced and discussed.
Validation. (Only applies to papers that contribute some solution to a problem; a counter-example to this are empirical study papers.) The paper needs to validate the presented solution, for example, using a theoretical validation with theorems and proofs, or an empirical validation in which the solution is applied to realistic instances. 


There is a lot of variation in how these criteria will be interpreted specifically. Some venues will have much higher standards than others, with top international conferences and journals on the one, and small national workshops at the other end of the range. Some venues will offer special paper types (for example, experience reports or tool demonstrations) that de-emphasize some of the criteria.