I always get somewhat upset when people misrepresent the doctrine of hell, though it is common, and even understandable. However, the arguments against it fail to recognize key facts, and often rely on hyperbole, representing hell as "unjust".
This particular tactic falls short, however, because it appeals to a sense of justice above God, one which He must hold to. In other words, it appeals to an objective morality beyond both human and divine, something for which there is no basis, no argument other than emotional appeal - especially because the sense of a morality above humans is often used to point to the existence of God. To say it points to the existence of a being above God is an entirely arbitrary step with no evidence, that can only be used to condemn practices you're not personally happy about.
So the idea that hell is unjust because God is violating some superior standard is easy enough to disprove (and the notion that it is bad by subjective morality is self-contradictory). A more cogent argument, however, would be to argue that it is inconsistent with God, as He is described as loving. And what loving God would punish people?
C. S. Lewis provided an excellent response to this in his book Mere Christianity, as a natural extension of his using the moral argument for the existence of a deity. He said: 'The Moral Law does not give us any grounds for thinking that God is "good" in the sense of being indulgent, or soft, or sympathetic. There is nothing indulgent about the Moral Law. It is as hard as nails. It tells you to do the straight thing and it does not seem to care how painful, or dangerous, or difficult it is to do. If God is like the Moral Law, then He is not soft. It is no use, at this stage, saying that what you mean by a "good" God is a God who can forgive. You are going too quickly. Only a Person can forgive.'
Though he does not state it directly, something can be inferred here: hell is a natural extension of a moral standard greater than humans. If there is a law above us which we must obey, and we fall short, then there is no reason outside of those standards to assume anything other than punishment. To say that God would not condemn someone to hell because He is a loving God is to ignore His other attribute of a just God. The only reason we know that God has provided a way out of hell is that He has given us a specific path out in the Bible. There's no reason to assume that His love would completely supersede His justice outside of personal perspective. Punishment can be inferred from wrongdoing, salvation can be learned through the Bible.
So to say hell is unjust is essentially placing your own standards above God in some way. It's a rejection of authority, something I have talked about before.
One more thing that's important to note: people will often attempt to overstate the severity of hell, saying that people are "mostly good", and that "no one really deserves such a terrible punishment". Now, this is another example of placing your own standards above God. I cannot actually say how bad the punishment of hell will be. It is not my place to judge others, and it is even forbidden to make that kind of judgement. But what we do know is that the punishment will equal exactly what is deserved. So in the hypothetical (though impossible) scenario of a person doing only one bad thing in their entire life, they would only be punished for that thing, exactly as would be appropriate for the severity of the act. It is never stated in the Bible that everyone will receive the same levels of excessive torture regardless of the severity of their crimes. That sort of hyperbole is only used to discount hell, but has no place in Biblical doctrine. Rather, we are told that everyone will be judged according to what they have done, and that God is a just God. To say hell is more than what is deserved has no real grounding. No human could say exactly what punishment any individual might deserve, but we do know they will be punished exactly according to how they have failed in their standard.
I like to say that hell is just math, but heaven is grace. Hell is the logical extension of a morality above our own, and the punishment that might be received will be perfectly appropriate to the individual. Heaven, on the other hand, is only achievable through the grace of God, through the path He has given us. Salvation through His son, Jesus Christ. It's impossible for us to earn it, but that it is given to us anyway is a supreme example of love.