Good point ericj, and one that many people miss.
There are two criteria for closed phones: isolation, and containment.
Containment stops others from hearing what you hear. Absolutely vital for recording with a mike! Vital in libraries. Vital while watching TV with others who are not wearing HPs (due to delay and distance the non-HP listeners would hear something echo-like). Courteous on trains, planes, etc.
Isolation blocks outside sound. Great for a dorm room.
Closed circumaural phones tend to do both of these, but not always. Denon D2000s do not isolate, for example. Some closed supraural phones do a pretty good job too, of containment anyway.
But the sound quality of closed phones will suffer, especially frequency response since there is nowhere for the displaced air to go. The top end studio phones from Ultrasone and beyer seem to have beaten this to some degree, their closed phones are pretty amazing (and expensive).
But in the end, closed phones have design criteria (containment and isolation) that cut against SQ.
Semi-open is a clever idea to get SQ back while preserving some containment, so they can be used in studios in front of mikes. It actually works. 600-ohm beyer 880's are among the best HPs on the market from an SQ standpoint, and they do see studio use (I am told).
Nubster -- "Enclosing" the ear is about circumaural vs supraural, not open vs closed. You had the concept right, almost: the best phones will be circumaural, but open and not closed.
Snap on a pair of K701's -- about as obviously open and circumaural as you can get -- and see how great they are.
