Not all mystics are Christian, though - I agree it can cause some taxonomic problems, but generally I would see mysticism as more a method and approach, and it simply seems to make more sense to have it under a more neutral category like Paganism.
that's a good question photon .... mysticism cuts across almost all religions in one form or another, christian mystics (the gnostics), jewish mystics, the sufi's, the druids, etc. it doesn't seem to fit well under paganism as a category. where would you place it photon? I'm not sure myself .... maybe 'alternative spirituality'???? hmmmmm, poh