The main dungeon in Slash'EM is much longer than in NetHack, and it incorporates many new special levels. One of the most exciting is the Guild of Disgruntled Adventurers, which is full of "player monsters" (archaeologists, barbarians and so forth), often equipped with offensive wands.
Gehennom is shorter in Slash'EM than in NetHack, consisting of 17 levels in Slash'EM and 20-24 levels in "vanilla" NetHack. It also has a special level for every demon lord (Demogorgon, Yeenoghu, Dispater and Geryon do not have lairs in NetHack and only appear when summoned). These factors combine to reduce the tedium of trekking through repeated maze levels.