Man, I hope this thing's still going....
But here's the most important aspect of my headphones/earphones:
The most important aspect of my headphones is the ability for it to transpose sound as naturally as possible. What I mean is, I want to listen to music as if I were rocking out at a concert, or calmly enjoying orchestral music in a chamber. In any case, the realism of an appropriate soundstage, and the natural sound of the instruments and vocals are the most important.
However, that's not say that the manufacturer can't add fun into the sound. Often times, recordings don't actually mimic live performances. Thus, I like my headphones to be able to emphasize the lacking aspects of music, whether it's weighting the bass, or coloring the highs, I want to be able to close my eyes and not be able to distinguish between a live performance or my music.
Another important factor is aesthetics and build quality (though the latter is more preferable). What I do not care about in headphones and I care about in earphones is how they will appear in public, because that's where I will tend to use them the most. Call me a audiophilic hypocrite, but at least having a positive public appearance will be more favorable than lugging around massive headphones. Hopefully this cleared things up on that front.
Lastly, when it comes to deciding upon headphones or earphones, I like to think of the contrast only within soundstage and airiness. Due to recent technologies, Custom IEM's and even some universal IEM's are able to mimic the sound quality created by even high end audio equipment. The only thing that is lacking is the soundstage, and so, when I'm looking for portability without staking sound quality, I reach for earphones. When I want airiness and such, I reach for the headphones. Hope this helps you make better and better products, Phiaton.