How do you have a single definition that includes both the Westboro Baptist Church and Zen Buddhism?
Rather quickly, and without thinking about it much, I think that I intuitively responded to your question, and intuitively have an answer: Each of those positions, or orientations, require specific metaphysical presuppositions. They are founded on metaphysical suppositions. And they are attempts to respond to and to answer what are perceived and understood as the requirements of those presuppositions. And it is with that, and in that, that I would locate 'religiousness'.
I do very much agree with you though that a definition of religion is, as you say,
nebulous. Starry. Vague. Hard to discern. In my way of seeing things everything hinges in that: It is essentially an existential and metaphysical issue. But I would also assert that even one who denies metaphysics ... is still very involved in metaphysics. Willey, in the post above, quotes Hulme who writes that in order to grasp our situation we require a 'master metaphysician'. It is an intriguing and challenging idea.
I'd also suggest the range of engagement people have with religion means that thinking about it in terms of an existential platform isn't going to capture all people. Some people go to church every Sunday and have done for 20 years and have never once wondered about why they're religious or what it means to them, any more than they wonder why they buy their food from a given shop. For them the simple habit and ritual of church going is all that there is in their religion.
What you say is very true. I would again make a reference to the thesis of Peter Berger who delves into the sociology of religion. I am more concerned and interested in the folks who are or become religious than in those who seem to avoid it. And the trend toward religiosity---despite modernity, despite secularism---indicates that, contrary to expectations, 'it' is still very much alive.
I also tend to think that even those who are not outwardly religious---as in practicing a specific liturgy or consciously holding certain beliefs and ideas---still function from an 'essential religious platform'. I realize that I cannot back up this statement with fact. It is an intuitive position I have and am trying to fill out.