Dodo_David said:
If a spiritually-dead person doesn't believe that Hell exists, then that person will be unmoved by any preaching about it.
Besides, preaching about eternal separation from God doesn't require a person to single out a particular sin for special condemnation.
Here's where I disagree with you. Everyone believes that hell exists even if they deny so on an intellectual level. When you get past the academic exercise, there is a deeply seated knowledge that everyone has that they will one day stand before their Creator and give an account of their lives. In this sense, there really is no such thing as an atheist. Furthermore, I find a strong correlation between the reticence of Christians to preach about hell and people acting in such way as to go there. Pastors who teach on hell are competing against more "seeker friendly" churches that don't judge, don't condemn wickedness, and don't instill a righteous fear of hell fire.
Genuine concern for the soul of a sinner, something lost in modern evangelization, gets through all the intellectual flak and causes a person to pause for reconsideration that they might be very wrong about God and about eternal life. It pierces through intellectualism and raises an appropriate fear in the sinner that they need to get their life right before the Almighty. Moreover, I think you're wrong that it's useless to point out particular sins. Homosexuals know on a visceral level, perhaps more than the average sinner, that they are living an odious lifestyle for which they will one day give account. If particular sins were unimportant, than we wouldn't be informed by the Bible that adulterers, fornicators, blasphemers, and the cowardly will not see the kingdom of God. Special attention wouldn't be given to the penalty of a homosexual life in the first chapter of Romans.
The problem isn't that people won't believe in hellfire, the problem is that we have been intimidated into silence and no longer preach it. And such a taciturn omission contributes to the ruin of souls.