For me, "portable" means using outside, while walking around amongst traffic, on the bus, subway, or other form of transportation in which I am a passenger. That being said, I do not evaluate portable headphones in the same manner, or place the same priority levels on their attributes as with fine, home headphones.
So outside, I need:
1. Closed or sealed - allows lower volume levels which is good for typically underpowered portable sources, and saves my hearing for old age. Also, leaking open cans are just plain inconsiderate to others on public transportation.
2. Efficient - I don't wanna carry an amp, so they must be powered directly by my portable source.
3. Light & small - can't be too obtrusive when worn, and should fold into something that will fit in a backpack along with a laptop, books, and whatever else I'm carrying today.
4. Not white - I have a fear that white headphones indicate iPod ownership. I don't want to be accosted by someone looking for something I don't own.
5. Easy on/off - People wanting to talk to you, quick listeing to subway conductor announcements, etc. necessitate this.
6. A little extra bass response - helps attenuate external noise, requires less volume for satisfying sound, and the extra bass is less obvious when "out and about". Detail is not the most important thing in this application.
Koss S/Portapro/KSCxx's are my portable favs, except for #1. Senn PX100's are open too. Ety's (and other canal phones) are awesome, except for #5. Senn PX200's need an amp.
So, I am currently using AKG K26P's. They are super-efficient, capable of sounding way-loud out of even my MDP. Plainsong's review on their sonics pretty much mirrors my experience with them so far, so all I will say is "ditto". Looking at them, no way people think I have an iPod with these. And they fold up nicely (if a bit tricky).
They weren't easy to find (Musician's Friend just got them in stock), but since they meet all of my criteria, they were worth the effort.