Pro Skill-based class system improves replay value
A character has seven selected skills, which effectively define that character's class: they have access to every skill selected, and no others. The base game contains dozens of skills, with many more available through both mods and official expansions.
Pro Several community mods were bundled as a free official expansion pack
In a testament to the quality of the Dungeons of Dredmor modding community, one official expansion pack consists almost entirely of user-created content. It was released for free, since they're not total capitalist jerks.
Pro Configurable difficulty accommodates both casual and hardcore play
In addition to basic difficulty adjustment, the player can choose whether death is permanent (in classic roguelike style) or merely resets the current level. There is also a mode called "No Time to Grind" where experience points are increased from all sources, but in compensation, levels are smaller.