Hmmm, i would say appearance matters. I always loves those products of great build qualities and aesthetic appearance cos i am spending like hundreds or bucks for an IEM. So, i really dun like the PK1, though it has great sound quality, it looks like sth you can pick up for 10 bucks. On the contrary i think ATH is a model of the industry, their phones are excellent on lookings. no matter how it sounds, you will have this kind of feeling, "seems it worths the money i paid". Appearance does come as important as sound quality most of the time.
Sound quality wise, i would say i prefer a rather warm signature. I am no near a gadget geek as i bought my earphone for enjoying music. So those super-analytic and cold sounds i.e. ER4 are not right for me. However, this is not to say detail is not important as if i cant find detail in the IEM i'll again feel like my money is down to the drain. So to give me the feeling of that my money is well spent, this IEM must be pretty analytic.
I believe that intense mids, punchy(can be a little flabby and bassy to create the speaker feeling) bass, slightly distant high is my favourite. As this will create a spacious feeling which you will not be tired of. Denon all series and ie8 i think are on the right track. However, IMO, i think, they are a bit idealistic for the tuning. The reason is most of us used IEM just for everyday use. So we want to enjoy them for pop,folk and so on. The signature of above mentioned phones can hardly be appreciated by average public or even some philes as they sound wierd for those genres. So i think it is good to find a balance.
Anw, to sum up. To lure me to pay for the hefty price, the Phone must have at least the followings.
1. Look good and expensive
2. Reasonably good detail in sound, a.k.a sound expensive
3. Brings me music enjoyment
3.2 warm signature
3.3 strong and not so distant mids,reasonale amount of punchy bass + good extensibility of highs.
This is, IMHO, the guideline of an ideal IEM should have.