Too easy. The hardest things I found so far are the barrels (only thing that managed to kill one of my low HP sorcerors) and crashes.
This is probably because we have all the time in the world to consider our options. There's no urgency as there was in Diablo to swap out spells or items, or worry about who we should be attacking and who is in the way. May want to up the ante overall (without increasing experience) to balance. The most difficult parts I find are killing the Butcher (Warrior only: the other classes can find grating and abuse as in the real game) and upon entering a new floor (where there are upwards of 20-30 enemies lying in wait).
Rogue, especially, appears to have the fastest attack speed, and so kills a lot of enemies before they can get close at all. I think the game needs to start factoring in side-stepping if it doesn't already.
Too few lightning spells. I managed to obtain Flash only - and too late in game to make any use of it.
The big item missing is Charged Bolt, and I imagine it's one hell of a spell to code (because the attack is kinda slow and should move across the map as time passes). Lightning is probably around (or if it isn't, the fact that we can't get to Catacombs is reason enough not to need it) and the others are too late-game to be seen. As a matter of fact, the Sorceror's starting staff should contain Charged Bolt charges.
Inventory should not close after dropping items.
Seconded. In addition, if it's not already on the big list, you should consider building the 4x10 slot inventory screen. There's no need to include pictures of anything (this is a roguelike, after all) but there could be different colors for used/empty and have white borders to represent "whole items" (armor/staves/bows are 3x2, shields are 2x2, small weapons 3x1, etc).
As a short-term addition, however, I'd like to suggest a way to know what our current inventory space is (ex: "[space used]/40 slots" in between equipment and inventory): it's just easier to maintain.