Great topic!
I sometimes have trouble articulating to other people why I enjoy LZ albums so much - it's really hard to give a distinct, clear answer. Their live performances obviously take on a whole new dynamic, but a Zeppelin studio album takes you to a unique place as a listener. I can only describe the experience as otherworldly, yet somehow simultaneously intimate. "Over the Hills and Far Away" is a great example of this - IMHO, one of the most beautifully crafted songs ever produced.
As someone who's not so well-versed in the technical aspects of music production, I can only rely on my gut instincts to tell me when something's good... and as a producer, Mr. Page has yet to let me down.