'The Moor' is the first Opeth track I ever heard, and damn it's good.
Yes, it's a boggy area as described earlier in the thread, but somehow the swedish word I associate with it is 'mosse', and it does go very well with the Opeth nature theme.
As for why it's mentioned so much in the songs - it's a concept album no? The Moor would be a landmark in proximity of the village/town. A spooky eerie place, perfect Opeth music scenery, myths and tales circulating about it in the village.
I wouldn't think of it as a majestic place though. More on the picturesque side, gloomy, mystic when shrouded in mist, dry gnarled bushes with small leaves on dry sunny days, and always desolate, lonely.
The song seems to be sung from the perspective of an outcast. Beaten and left to die on the moor, and then wakes up as a ghost or from a trance of some kind.
All speculation of course. It would take a small book to describe exactly what visions the Moor gives me. =)