When submitting a manuscript for possible publication by a review, we usually asked to justify that our manuscript contains sufficient contributions to the new body of knowledge.
This is a really critical question Mounir, and if not properly addressed will lead to a rejection. I have heard the problem summed up as the "so what?" question. In other words, I have just read your article and -so what?
Have I learned something new, or different? Has my understanding of a problem been improved, changed or challenged?
I see it as the authors job to inform me right from the beginning of the article about what is new or different and about what I will learn from reading the article. In short I expect to be told what the paper will contribute to the field.
Of course there are different types of contributions; descriptive , theoretical or even methodological. Typically descriptive contributions have a lower impact than conceptual, but often a good paper will use the descriptive contribution to provide an analysis which becomes the contribution to theory.
However, in my view, the crunch point is that a paper which fails the SO WHAT test will not be accepted by a good journal.