Most of the game is spent replaying the same dungeon, over and over again (there are bonus dungeons, but they're not really essential to the main quest). The layout changes, and depending on how far you got last time, the game might add extra floors. Keep in mind that you never "have" to die to progress the story. Reaching the end of the central dungeon is considered a death, so you don't have to leap into a spike pit just to get ahead.
The greatest incentive to keep playing, for me, was seeing how the peculiar characters that inhabit the overworld and dungeon change every time I play. For example, an NPC you find on the 5th floor might come across as a gibbering madman the first trip through, but his dialogue expands with every visit, revealing extra snippets of his sob story. In the absence of a really strict narrative, I found that to be a pretty clever way to add character depth.