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).
In talks, you can use some didactic means. For example: start by presenting the problem. Then, ask the audience to think about it for a minute and suggest some directions for a solution. If somebody suggests a direction which doesn't work, show why it doesn't work. If nobody answers, prepare in advance some apparently useful directions that turn out to be unfruitful. 

The disadvantage is that it requires you to spend about half the presentation time on discussion with the audience. The advantage is that, maybe, after the talk, your audience will better appreciate the difficulty of the problem.

I am not sure how to adapt this didactic technique to a paper, though.