Page actually did some really great stuff, harmonically. Especially in the later years when they got away from all the blues based riffs that dominated their music. Kashmire (sp?) was great, plus songs like the Rain Song, Song Remains the Same, etc...I believe Page wrote the intro to Stairway with an A minor tonality in mind utlizing ascending and descending motion to enhance the progression.

A dorian is a good call, although there is some elements of A natural minor in there too. I just think it's misleading to label that 1st chord C6/A when the tonality is definetely minor. D maj. could be explained w/Melodic minor, but that's a long shot, and I'm not going to go there.