My thoughts so far on The Hell:
Item's stat requirements were increased a bit too much, and strong stuff drops WAYYYYY to early. Case in point, the first weapon I found on dlvl 1 required something like 60 strength and 40 Dexterity. That means it would have taken me a minimum of like 10 levels putting points into nothing but STR and DEX to wield it. Even now, I'm in the catacombs and my character is level 13. No way I can have any hope of wielding this shit that's dropping with 130 req strength/magic/dex. Leather fricken armor should not require a 100+ strength hulk to wear.
This also applies to spells. 45 Magic to cast Town Portal is a PITA, though eventually you can get most of the magic from items if you want nothing more then the low level stuff.
Items sell for way too little. Even the strongest item that's like 20 character levels beyond me sells for 1/100th of what any random magical item in the base game would be worth. We're talking hauling back a full inventory of items and getting a total of around 150 gold. Keep in mind that to buy a single potion (which restores about 20-40% now), you need 100 gold each, and identifying an item costs 100 gold each. If it weren't for the fact that my assassin has identify as a character skill, I would be broke. As it is, I've been able to buy one (1) item so far and I'm almost halfway through normal difficulty. And that item was just to boost my magic stat so I could finally cast Town Portal and Heal... also I found this Fury spell that temporarily increases my strength, dexterity, and vitality which is pretty damn awesome. Too bad it doesn't count for equipping weapons.
I think playing a full caster would be murder. It seems like 95% of enemies are immune to at least magic and lighting, with a resistance to fire. There are a lot of triple immune enemies as well. Do they expect sorcerers to melee or something? Regenerating magic is nice but if they are immune you can't really do a thing.
Some/most the quest bosses are ridiculously overpowered. Butcher has something like 8k health and will pretty much one shot me half the time even now when I'm at level 13 with over 180 health. He also spawns on dlvl 1, that's just evil. For comparison, the base butcher has like 100 health and might do 15 damage, spawning on dlvl 2. The random unique bosses are pretty well balanced, though. Very tough fights, having about 3x the health and 2x the damage and a strong horde behind them. Guaranteed strong magical drop too (though keeping in with my previous complaint, it usually ends up TOO STRONG).
I don't like some of the sounds and music that were taken out, but I believe this is actually Hellfire's problem so I can't complain. At least the footsteps should really be added back in, that can't be too hard.
Other then that, it is indeed pretty awesome. I just think that not being able to buy much stuff and the equipment problems tilt the balance against certain classes a bit too much. In fact, I think that if I had been playing anything but the Assassin I would be having much, much more troubles then I am now. Their dual wielding makes you incredibly strong even with sup-par equipment.