Well firstly there will be defining and non-defining characteristics for religions. The prior will be things that all religions share but are not present or at least not abundant elsewhere. The latter will be things that all religions have in common, but refer to things also found elsewhere.
There will be a whole host of things that are non defining characteristics that all religions have in common.
Defining characteristics (common yet exclusive to religions) will be far less numerous.
Further to this I doubt that its really useful or insightful or even very fruitful to search for some solitary characteristic that is present and defining of all religions.
Because religions are heavily integrated in human life, social and cultural circles it is inherently quite complex entity. Rather I think the question is what pattern of characteristics seem to be present across all religions, thus defining them so. So any one characteristic is not in itself exclusively descriptive of religion, but its relative expression in relation to a number of other identifiable characteristics will be unique. This way you can construct a sort of bar code for what is common among all religions.
(High five for unnecessarily complicated replies that dont quite answer the question! )
Good points, though.