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).
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> To make things worse, sometimes the author is unable to illustrate the application of the idea by sufficiently striking examples.
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I think this goes hand in hand with the problem you describe. You cannot expect an audience to evaluate the difficulty of the problem you have solved in a short time and often the same goes for reviewers and editors (unless of course, they are familiar with the problem). But you can show that you are addressing an important problem, which does not yet have a (satisfying) solution.

Now, if the latter is true, others should have thought about it before you. But as they have not come up with a solution, the problem could not actually have been that obvious. (Of course, it could have just been that nobody spotted the problem or the general approach you have taken – but then you can take credit for that.)

 I have attended some interdisciplinary conferences and observed a general tendency that participants from more theoretical fields failed at motivating what they are doing – which was one of the key issues that made their talks very difficult to sit through or their posters not interesting.

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If there are standard approaches to your general type of problem, you could also shortly explain why these do not work. This way you can demonstrate that the problem required some thingking out of the box.