A key move in standard Leibnizian cosmological arguments is the claim that:
(UCB) The universe -- or (if the universe doesn't exhaust physical reality) all physical reality -- is a contingent being.
Now the primary means of support for UCB is a conceivability-possibility inference. Richard Taylor's use of such an inference is representative in this regard. Thus, he argues that for any object in the universe, we can imagine that it fails to exist (e.g., a six-foot-in-diameter translucent sphere). But if imaginability is evidence of possibility, then this is evidence that for any arbitrary object in the universe (whether a stamp or a solar system), it's possible for it not to exist. But we can just as easily imagine the whole universe failing to exist. Therefore, we can say with equal justification that the universe can fail to exist, in which case it's a contingent being.
Is the line of reasoning above for the contingency of the universe a good one? One might think not, on the grounds that Taylor conflates evidence for the possible non-existence of a material object (a stamp, a solar system, etc.) with evidence for the possible non-existence of the stuff of which it's composed (matter-energy).
William Lane Craig is aware of this sort of worry. However, he thinks he can get around it and make legitimate use of a conceivability-possibility inference to support UCB by cutting to the chase and asking us to imagine the most fundamental constituents of reality -- quarks (assuming the string theorists are wrong) -- failing to exist; alternatively, he asks us to imagine a universe composed of different quarks. Given this modification of the thought experiment, he assumes that we can adequately imagine this, and further that this is sufficient prima facie evidence that such things are possible.
I've raised worries for Craig's defense of the Leibnizian cosmological argument on other occasions. Here's another one: Craig's defense of UCB makes no real advance over Taylor's. For it seems to assume a "thing" ontology about fundamental reality, while a "stuff" ontology of fundamental reality is epistemically possible.
Let me explain a bit. For simplicity's sake, suppose there are just nine things (quarks, say), and these, in turn, are composed of a more fundamental "stuff" (matter-energy, say). Suppose further that the latter is a metaphysically necessary substance. Finally, suppose the stuff is capable of an unlimited number of modes of existing (e.g., as nine particles, as 18 smaller particles, as one big particle, etc.). If so, then while the particles -- the things -- are contingent beings, matter-energy -- the "stuff" -- is not. But if this scenario is epistemically possible, then as with Taylor's defense of UCB, Craig's defense of UCB fails to rule out that he's conflating the contingency of things within the universe with the contingency of the stuff of which it's composed. And if that's right, it doesn't seem to me that Craig's defense of UCB makes an advance over Taylor's.
Now one might try to push Craig's point here by saying that we can adequately imagine the universe (or a universe) as composed of different matter-energy, but at the very least, this isn't clear. Indeed, I don't even know what I'm supposed to imagine here: what would it be like for there to be a different kind of matter-energy that could, in turn, be used to compose material things?
In short, it seems to me that Craig's recent defense of UCB fails to rule out the epistemic possibility of a stuff ontology of fundamental physical reality. Because of this, his defense of UCB fails to rule out that he's conflating the contingency of things within the universe with the contingency of the stuff of which it's composed. And because this was the worry that Craig's defense of UCB was supposed to avoid, it fails to make a significant advance over Taylor's defense of it.
 Joshua Rasmussen has suggested to me that one can also imagine the universe as composed of more or fewer quarks, and that this in turn might confer sufficient justification on UCB. Below I'll raise an objection that challenges these thought experiments as well.