Sam’s Hell is reminiscent of how Mary and Jess died. His personal Hells on earth started with fire consuming first his mother, his home, and then his lover and their home. His chances at the life he wanted for himself. They died because Lucifer and Azazel were trying to get to him. Perhaps Lucifer simply had to remind him of this when they were in the cage and then could just sit back and watch while Sam burned himself up with his unjustified, self-imposed burden of guilt and unfulfilled dreams.
Dean’s Hell is bloody and confusing and exposed and confining and solitary, just like his biggest fears in life. We see him shouting out for Sam, and we know that despite Sam’s best efforts, he can’t reunite with Dean. They can’t be together. Sam can’t be there to help Dean control that spider web of conflicting impulses he carries in his head. He is missing his protective layers of clothing and is stretched out in a mass of raw nerves. So Alistair just has to keep cutting away at Dean’s defenses until he finally gives up on Sam and gives himself over to all the pain he keeps chained up inside his own mind. Dean will torture himself in the forms of all those souls he cuts up and he’ll enjoy it, and he’ll hate himself all the more.
The Hell Crowley shows Cas suits both their sensibilities. Crowley knows he doesn’t need to go to such showy extents as Lucifer because he knows that people left with too much time on their hands will spend it regretting everything they did to get themselves in that spot. Physical pain would only distract them from their guilt and sorrow. And he can show this Hell to Cas without reminding Cas of fighting through Dean’s Hell to save him. Cas won’t have that natural revulsion fire and brimstone and screaming would have caused. Cas will be able to tell himself this partnership is for the best. Crowley is quite clever and knows who he is dealing with and all he really cares about is the power he’s getting anyway. Cas is willing to be fooled by it because he wants that same power.
Bobby has a stereotypical Hell. He’s an older guy and probably was raised with this sort of imagery. This is the Hell that’s most likely depicted in all those books he knows so well. It’s full of other people suffering physical pain and humiliation who he can’t save. His personal pain is caused by being faced with an infinity of demonic Sams and Deans who come to taunt him. In his nightmares he dreamed of his wife blaming him for not saving her. His surrogate sons taunt him in Hell with the fear for them he felt when he died, the reason he wouldn’t move on when the reaper came. They taunt him with the possibility that they’ll be lost after he left. He sits in his unlocked cell day after day waiting for them to come and show him that his greatest fears were realized while he’s been stuck down there in that labyrinth of other people who cry out for help he can’t provide, and he scratches marks on the wall so he can keep track of it all. So he won’t forget. So he’ll stay sane enough to keep punishing himself.