If you had to find one value that the majority of Pagans would identify as a Pagan value, the sanctity of the earth might be it. For many Pagans, this value is central to our spirituality.
Some say that the earth is the Goddess and don't mean that metaphorically (some do). Many subscribe to something like the Gaia hypothesis, which posits that the earth itself not only houses living beings but is a living being itself. Many say that we find the gods in nature. Pagans tend to be animists, believing that even things like trees, rocks, and mountains have a kind of soul, spirit or consciousness. Many if not most contemporary Pagans believe that the earth is holy and has instrinsic value.
In keeping with this value, Pagans strive to live in right relationship with the earth and her creatures. We invoke the ideas of balance and right relationship and reject models of dominance. We may practice permaculture, buy organic foods, garden organically, strive to live sustainably, belong to conservationist groups and land trusts, advocate rights or protections for animals, or shelter and rescue domestic animals. Most of the Pagans I know are involved in one or more of these practical, earth-honoring activities.
Our rituals often take place out of doors, and even when they take place indoors, we often invoke and honor nature, the earth, or earth spirits and guardians. Many Pagans honor the old agricultural cycles and the phases of the moon. We practice grounding ourselves and our energy, and we value the particular places where we live. One Witch I know says he can't sleep well if he doesn't know the land.
Some conservative Christians make the disingenuous mistake that all environmentalists are Pagans. Of course that isn't true; one can value nature, animals, trees, habitat, and the wild without practicing Paganism. But Pagan religions are the only ones I know that make concern for the earth a central spiritual value.
Surely the image of earth as Mother arose in cultures where there was less separation from the land than in ours, and where breastfeeding children was the norm. The way that a human mother gives of herself for her child, providing nourishment and care, fulfilling the needs of early life, mut have struck our ancestors as analogous to the way the earth provides us with water, food, medicine, shelter.
In right relationship, we love the One who sustains us, as She sustains us with her love. Blessed be.