Yeah, the LCD-2 has one of the smallest soundstages I have heard in an open back headphone.
Thinness of the mids? Certainly not, at least I think so. I do find the mids to have a slight metallic kind of sound to it though; I still like the mids nonetheless. :D
I don't know if it's due to the K 701's soundstage, but I find that a good chunk of my music doesn't sound very natural with the K 701. Maybe it's because it's open-back? Maybe it's because the K 701's strong L/R imaging leaves the center kind of lacking a bit to me? I didn't hear this kind of sound with closed-back headphones, such as the Shure SRH940.
Not to say the K 701 sounds bad, it's currently my reference headphone.
I think you're right with the strong L/R imaging maybe giving the impression that the center image is lacking. But this can only be in comparison to other hp's with a more intimate soundstage where the center image is being kind of 'forced' on you. I personally think that the center image is good. The K's are my prefered hp for piano now, and they used to be when I ownded the 702's too. And you have to have a good solid center for solo instuments. Especially piano, for me its one of the hardest instruments to replicate with speakers and headphones. The dry and free sound of the K's for some reason is brilliant at this. Any colour in a headphone can affect the speed/decay and congest complex keyboard peices. There seems to be not much added and not much lost with the AKG's.