Pretty serious bug: If you save and quit during the death animation, you can reload your game and become a "ghost". You'll be invisible, have 0 health, can't spend any energy and you'll be followed by all of the monsters. However, if you can make it to the stairs (or out of the level, in the campaign) then you'll be revived!
I wonder what happens if you have the oracle's passive healing ability... I'm too lazy to build a character that high just to kill it off though. :P
Another thought: Starting as a warrior is really tough. You have to take two additional class levels before your starting armor stops slowing you down, you can barely hit anything with your starting weapon, and you don't start with a ranged weapon. It's a much better idea to take 2 levels of rogue before starting down the path of a warrior. Heck, even just the first level of rogue, with its starting equipment is way better! How about giving warrior a blue +1 first-tier weapon to begin with? If you don't like that idea, maybe put rogue as the first option when creating a character to help offset the initial perceived difficulty of the game.
A few more suggestions:
1) Make potions usable from the inventory, and stop putting them on the action bar by default.
2) Make the action bar scroll vertically instead of horizontally. I never use the non-visible boxes except for permanent buffs because they're a hassle to get to. (Also, I'm always really worried that I'm going to click the first/last box as I'm scrolling.) I'd really like to be able to have one bar for buffs, one for summons, one for attacks, etc.
3) Show which function key each action box is mapped to. Perhaps in the corner of the box?
4) It'd be nice to be able to see the log after dying. I just got one-shotted by a manticore+, and I would have liked to see why. :(
5) I wish I could see my summons' stats, and know how to improve them. Summons seem really weak at higher floors, even with summoner 5.
I'm really enjoying this game. I got up to floor 23! I've noticed a couple of bugs though:
1) Smite doesn't consider the lich and the various vampires as undead.
2) Over time, the UI starts to get really laggy, and sometimes the screen doesn't redraw correctly when switching menus. Closing the menus and reopening fixes it though. After this starts happening, when I close the tab, the Flash plugin stop responding. (I'm using Chrome 36 in Windows 7.)
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 "1Quest"){kind=icon}
 "Phobiaphobiaphobia"){kind=icon}
Thanks for your comment, I take notes of all this suggestions!