In the Calvinist/non-Calvinist dialectic over whether or not God wants people to be saved, the following often occurs:
Claim: God doesn’t want some people to be saved, on Calvinism. This is because he could cause them to believe if he really wanted to, but he doesn’t.
Response: But you have the same problem. On your view, God still has the power to cause people to believe, but he doesn’t, because he values free will. We have the same kind of response: God could save everyone, but he values something else more. For you, it’s human freedom, and for us, it’s his own glory.
It seems to me this response misses a crucial nuance that I’d like to explore.
When the non-Calvinist asserts that, on Calvinism, God doesn’t want some people to be saved, what she is really claiming (or at least what she should be claiming) is that God doesn’t want some people to believe. It is this key distinction in the dispute that will make all the difference. What does it mean to “believe” in Jesus? Does it mean to believe in his existence? Well, sure. What about his claims to divinity, and his resurrection? Of course. But it also means much more than these mere intellectual states. It is an active trust in God for life and salvation (just ask yourself—or a Calvinist—if someone is really a believer who has no interest in trusting God or following Christ at the time of their supposed conversion).
But this kind of trust, love, and discipleship can only be entered into freely. I suspect that, at bottom, most Calvinists would agree (on certain conditions, no doubt—but conditions not needed at present to agree). Non-Calvinists in the debate tend to believe in the thesis of incompatibilism—that causal determinism is incompatible with freedom—and as such, causal determinism is incompatible with freely entering into a love and trust relationship with Jesus Christ. Calvinists often, though not always, affirm compatibilism—the thesis that causal determinism is compatible with free action—and so causal determinism will be compatible with freely entering into such a relationship.
Now we can see the difference between the initial claim and the response, and why the response (given by people such as John Piper) doesn’t have the intended force. It’s because the answer is “no”—God cannot force someone to freely do something, any more than he can create a married bachelor or lie or will himself not to exist, etc. The whole idea of salvation presumes a free choice to participate in sin such that one needs salvation.
Now on the non-Calvinist view, God could have created creatures without any free will, such that none would ever go wrong. But then these wouldn’t be humans (and plausibly wouldn’t have been made in the image of God); they would be something else entirely. Notice the response loses force if we amend it to, “Yes, but on your view, God could have refrained from creating humans and no one would be lost;” the lack of creation is not symmetrical to securing universal salvation. The fact remains that on the Calvinist view, God could secure a free response of salvific belief for everyone, and simply does not do it.
 What I am calling “the Calvinist view” is shorthand for the view of Calvinist theology with the thesis of causal determinism. Thus, in principle (and in practice!) there can be Calvinists to whom this does not apply.