Put "Hell" into a Google image search and you're likely to come back with something like this.
|Lots and lots of fire.|
I'm not going to be that smug jackass who pretends he's read anything by Sartre. The only thing I have to offer is that one famous quote from No Exit (had to look that up, I admit) "Hell is other people." It's a start. It's sensible. Probably all you need to make a very good Hell is to let a bunch of people loose and let them do what they would. But "other people" doesn't take it as far as it can go.
Hell is variety. An unending lake of fire is dull and is the kind of thing you're going to get used to after a while. Part of what makes pain really awful is the anticipation before your nerves start screaming. What use is pain without an end? There has to be a variety. When the imp comes to kick you in the balls next Tuesday, he'd be smart to jam his pitchfork into your knee and yank it like a lever. Not expecting that, were you?
And he's only going to visit you every so often at irregular intervals. There needs to be contrast between pain and not-pain. Because the second thing that makes a good Hell is thought. Imagine constantly anticipating that pinch. When will it happen next? Torture is bad enough. I'm not going to say that the constant expectation of it is worse, but it's absolutely a part of the mixture.
Lake of fire? No, today you're in the frigid swamps full of nasty poisonous things. Some of you will come back without eyes. Some of you didn't have eyes to begin with, because inequality is part of it. When the demons come and remove the feet of every tenth person, you'll understand why after carrying that person ten steps, or after leaving them where they are. Only one needs to suffer to make ten miserable.
On that note, this story is turning out a lot darker than when I'd originally conceived it. That's not a complaint, just an observation.