Slime Pits is fine now, IMO. Still the hardest of the hard-difficulty dungeons, but well within reason. The PQI has been giving me nothing but Slime Pit runs for some time now, and I've been having a fairly good success rate with them.
Demonic Library definitely needs the attention. It was too easy for a vicious scenario before, but it's swung much too far in the other direction. I also think the Avatar is a little bit underwhelming as a boss. I'd prefer DL to go back to how it was on the surface and just rework the final battle to be that much more of a challenge.
I've found that some dungeons (Hobbler's Hold, Den of Danger, Eastern Tundra, and Namtar's Lair do this a lot) have a nasty habit of placing the boss at the end of a long corridor filled with level 7's, 8's, and 9's. The big problem is that these corridors are often difficult to identify until exploration is almost complete, at which point you basically have a mini-matron on your hands if you don't have ENDISWAL. Still winnable (in fact, I pulled off a Namtar's Lair win as Warlord recently when Namtar was on the very last tile I explored and I had expended several potions beating level 9's to reach him) but definitely not a fun layout to have unless it's an intentional part of the dungeon design like Dragon Isles.
Labyrinth could use some attention. You're still totally dependent on finding some method of wall destruction to have a clean run on it. Don't know what the solution is, but something would be helpful. Maybe a "Bridge Troll" style opponent that appears near the player start location and drops a bear mace?
Otherwise, most dungeons are in a presentable state and any gripes I have are relatively minor and not worth taking focus away from these other problems.