Question: In nearly any field there is a number of important results which look obvious to experts *post factum* but somehow are not that easy to come by in the first place (e.g. in mathematics some important definitions look exactly like this). Unfortunately, this apparent *post factum* simplicity makes conveying the importance of the idea to expert audience (and in particular to the journals' editors and referees) very difficult. To make things worse, sometimes the author is unable to illustrate the application of the idea by sufficiently striking examples. 

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> The question is whether it is possible (and if yes, how) to mitigate this **apparent post factum simplicity** in the talks and research articles, i.e., what can be done to *adequately* convey the significance of "post-factum-obvious" results to the audience and, in particular, to get these results to the journals they truly deserve?
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I am particularly interested in the advice applicable to mathematics/mathematical physics but the suggestions suitable for other fields are very welcome too, as the situation in question does not seem to be all that field-specific.

Answer: 
The best approach may be to address the issue head-on: include a paragraph in the cover letter, in the introduction or discussion sections of a manuscript, and explicitly in the conclusions section of a talk, that sets up a "straw man" critique. Something like: this result may seem obvious because of (expectations in the field; the beguiling simplicity of the final proof; the incorrect assumption that the problem was already solved; etc). But actually that assumption is flawed (and say or show why). Our result is not obvious because (state reasons).
The OP writes that "experts", "journals' editors" and "referees" will *fail* to realize the "importance" of a result, if it looks "obvious" after statement.  

I note that "importance" should have been somehow defined in the question, or at least be endowed with some operational meaning. But let's pretend we understand roughly the same things by reading the word, and let's assume that, indeed, all these people don't get this "importance". Then *who* understands it? The author? Namely, the one who "is unable to illustrate the application of the idea by sufficiently striking examples"? In other words, the person who has labored deeply into the case, he, on the one hand cannot come up with "striking examples", but at the same time "knows" that his result is "important"?  

*How* does he know, if he does not know *why* the result is important? And if he knows *why* it is important, how can it be that he cannot communicate, however imperfectly, these reasons why? He may be lousy regarding presentation skills, he may be a bad writer. But still, how come *nobody* of those in the scientific community that know the field and the subject, realizes the importance of the result, in any form of communication, except the author? 

Important results are sometimes initially overlooked because they are stated in a very *specialized framework* and their generality and wider applicability are not immediately, or even easily, evident. But "obvious" results, are, exactly, evident, and so their importance should either be evident, at least to *some* individuals that are part of the populations of "experts", "journals' editors", and "referees", or chances are, it does not exist. Difficulty of derivation or conceptualization are neither necessary nor sufficient for "importance".

But of course, I may be wrong -so I would really, honestly, appreciate it if someone could provide an actual example of such a situation.

I note that the impression I got from the question is that the OP does *not* refer to "paradigm shifts" and "revolutionary ideas" that may "fall on deaf ears" for sociological reasons. For such situations, it is a whole different, and largely theoretical, discussion.