My analysis of the game going by topic:
+More customizable, more mod friendly
+Sleek interface design
+More character classes
+Secrets to find
+Bonuses to unlock
+Rating of how well you are doing (with score)
-Less character customization
-Game play may get repetitive
-Classes aren't differentiated as much
The action game is a good direction, however since DoP is the primary reason everyone is here, perhaps more rpg elements could be added? For example, finding gold and buying items/upgrades/skill training. Also, the score could be used as exp for character leveling. In order to force attention more towards the action part of the game and less towards the number crunching of rpgs, the bonuses gained from the rpg aspects should remain small and not significant enough to dramatically alter the monster to character power balance.
Another thought is that upgrading the graphics is a good thing, and perhaps may even need more upgrading. My thoughts on this is that RPG players a generally hard core gamers, and are more lenient towards graphics. They are more interested in story/number crunching/goblin bashing. Graphics are a plus, but not really necessary (as demonstrated by MUDs). However for the action crowd, gameplay and graphics are pretty much all that the game has. Storyline isn't as significant, while assuming a character to run around and do cool stuff with is more the point.
Fate has been mentioned in relation to Kivi's Underworld, and I think it is a misleading example. Fate is about number crunching and goblin bashing. The graphics and interface/character control is a nice bonus, however its main draw is unlimited dungeon crawling, item finding, and character enhancement. KU does not rely on these features, instead relying mainly on character control, graphics and interface, and how intense the action and challenge is to the player.
I didn't get very far in KU due to lack of time. However, since it seems to be more of a Zelda type game, I am hoping/assuming/suggesting that there be quite a few unique boss encounters if they already don't exist. Having only seen the first portions of the game I don't know if this is there or not, but if not I would highly recommend its addition. Part of Zelda's appeal, while being an rpg, was not number crunching and character improvement, but the story and boss fights (Alright, that is countering my previous argument of action games not relying on storyline as much..) are why it has left such an impression on the gaming market. And its uniqueness for it's time. Timing really helps.
-Map-based Level Selection:
One of the things I immediately thought of when I saw the level selection menu was that it would be cool if the levels were placed on a map. I'm a big map lover, as I like to be able to visualize where something is in relation to other things. In this case, it would be where the Ice Caves are in location to the first town, and where the Dragon's Lair is in relation to the Ice Caves, etc, etc.
Since this is underground (I am assuming, with Underworld in the name), perhaps some type of underground/cave system/network maps could be displayed, perhaps in some sort of pseudo-3d map to allow for locations that are on top of and below each other.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, & Din's Legacy