The first answer that popped into my mind was the last few lines of a poem by the iconoclastic Zen monk Ikkyu:
To harden into a Buddha is wrong;
All the more I think so
When I look at a stone Buddha.
To answer the question more completely, I think I have to recount my whole progression from where I was four years ago (which was, to recap, a panentheist progressive Christian who practiced Buddhist meditation) to where I am now (an atheist), because I think being a Buddhist was just a brief way station along that path.
One of the main things I learned about myself by living in a Muslim country is that my worldview is fundamentally a naturalistic one. My Muslim friends' belief in jinn and witchcraft seemed painfully off-base to me, not because I thought they believed in the wrong set of supernatural forces while Christians believe in the correct supernatural forces, but because I really don't believe in supernatural forces at all. I've never believed in angels or miracles, for example; I haven't believed in an afterlife since I was 12; and I don't know how many years it's been since I believed in a personal God in the sense of a discrete, anthropomorphic being "out there" who influences the goings-on of our universe. I nonetheless believed in some sort of ineffable divinity, and in a Western, mostly secular country, it's fairly easy to overlay some vague belief in the Ground of Being on top of an otherwise naturalist metaphysic and call that Christianity.
Encountering religious people (both Muslim and otherwise) who really fundamentally believe in the existence and power of supernatural forces, though, made me come to terms with the fact that I'm not one of those people. And, to be honest with myself, I had to admit that the kind of worldview espoused by the Scriptures and church tradition was also a supernatural one, despite the efforts of Tillich, Robinson, Spong, et al. to update the Christian understanding of God in the light of a modern worldview that rejects supernaturalism. It began to bother me that I spent half my time in church affixing mental footnotes to every creed I recited or hymn I sang -- for example, mentally noting that by "Christ" I meant "the spirit of compassion and self-sacrifice" rather than "that dude named Jesus who lived a long time ago." I got tired of the mental gymnastics necessary for me to affirm the things Christians affirm.
One day I sat down in the library to read Tillich's "Christianity and the Encounter of the World Religions" and read the sentence: "It is natural and unavoidable that Christians affirm the fundamental assertion of Christianity that Jesus is the Christ and reject what denies this assertion." Even though I've certainly read more thoughtful and well-reasoned expositions of the relationship between Christianity and other religions (most notably Diana Eck's in Encountering God), somehow that sentence was my proverbial straw that broke the camel's back. Even Tillich claimed that "Jesus is the Christ" is the "fundamental assertion of Christianity," and that's just not something I could assert without all those aforementioned mental footnotes. I realized that if I was finding Paul "The god of theism is dead" Tillich impossibly conservative, I should really stop pretending the wide umbrella of Christianity could (or should) extend as far as where I now stood. In that instant, sitting in the religion section of the CMU library, I realized I wasn't going to call myself a Christian anymore. The thought felt like vertigo, but I also felt the weight of all those mental footnotes lift.
I didn't leave Buddhism in the same instant because reconciling Buddhism with my otherwise secular worldview didn't require the same mental gymnastics. I always appreciated that Buddhism focuses pragmatically on how to alleviate suffering rather than on metaphysics, that the Buddha doesn't seem to have cared about whether there was a god or not, that the Buddha taught that we should test his teachings against our own experience and reason instead of blindly following set doctrine. Because of these things, practicing Buddhism as a secular person didn't feel dishonest or disingenuous.
But as I started to practice meditation more often and listen to weekly dharma talks, it dawned on me that Buddhism is hardly free of dogma. Its teachings on the afterlife are both as central to its teachings and as implausible as Christianity's, for example. And, in practice, the vast majority of Buddhists in the world practice a supernatural religion, whether they are superstitiously chanting the name of the Amitabha Buddha or undergoing body mutilation in order to channel the emperor-gods -- or offering food to the hungry ghosts, as I myself did at a Zen retreat 3 years ago. It's possible to practice Buddhism without these things, just like it was possible for me to practice Christianity without believing in an afterlife, but then we're back to mental gymnastics.
Ethics is a good example of this. I think the Five Precepts are a better guide to ethical behavior than the Ten Commandments. But why do I think that? I am measuring both sets of rules against my internal sense of what kinds of behaviors do and don't cause harm to others, and the Precepts seem like a better approximation of what secular ethical reasoning suggests. But then, if my fundamental yardstick is the utilitarian one, then I don't need the Five Precepts any more than I need the Ten Commandments; I just need to employ ethical reasoning. So why not cut out the middleman, as Sam Harris puts it, and do what I think is right instead of trying to find a religion whose moral code approximates what I already think is right.
On a deeper level, though, I think I stopped practicing Buddhism because I stopped craving the things that I had looked to it to provide. I was initially drawn to Buddhist practice because it seemed to hold out the promise of inner peace and the power to still your own chattering monkey mind. To a naturally anxious kind of person like me those sound heavenly. Another way of saying this is that I wanted to use Buddhism as a tool to help me become the person I felt I ought to be. (I think now that this is a very un-Buddhist reason to practice Buddhism -- although I'd still like to hear an answer to the question I asked of that Zen monk three years ago: isn't the desire to extinguish our cravings itself a craving?) Somehow, perhaps as I've gotten older and a little less insecure, I feel less need to fight the way my mind works on its own. I no longer feel like I *ought* to have one-pointed mind; I have monkey mind because I am descended from monkey ancestors, and on the whole a distractable psyche has served our lineage well. One day I realized that cravings and attachments bring me most of the joys I experience as well as most of the suffering, and that I'm actually not all that interested in extinguishing them after all. So I took off my dharmachakra necklace and prayer beads and stopped calling myself a Buddhist. I still have warm feelings towards Buddhism: my dharmachakra and fo zhu are still at the top of my jewelry box; I'm reading Confessions of a Buddhist Atheist right now; just yesterday I idly looked up the closest zendo to my new house. But I don't want to harden into a stone Buddha; I just want to be a flesh-and-blood Marjorie.
Which somehow reminds me of another poem I love, Mary Oliver's "Wild Geese," which I think comes closest to summarizing my current perspective on spirituality:
You do not have to be good.
You do not have to walk on your knees
for a hundred miles through the desert, repenting.
You only have to let the soft animal of your body
love what it loves.
Tell me about despair, yours, and I will tell you mine.
Meanwhile the world goes on.
Meanwhile the sun and the clear pebbles of the rain
are moving across the landscapes,
over the prairies and the deep trees,
the mountains and the rivers.
Meanwhile the wild geese, high in the clean blue air,
are heading home again.
Whoever you are, no matter how lonely,
the world offers itself to your imagination,
calls to you like the wild geese, harsh and exciting--
over and over announcing your place
in the family of things.