Normal levels on Dark difficulty have only one section instead of three (no killing all the enemies to knock down the wall and continue forward), but instead of increasing the number of low level minions they're replaced by three boss-type characters attacking you at once! Not too scary if you've been on Floor 90+ of Tower of Infinity, but it takes skill and/or weapon abiliites to make it easier.
Unlike normal bosses, these enemies have minion drop rates, so it's common to get 0-1 drops in a level; you had better have that full Inferno set before you get here, because I don't think grinding on this difficulty will make it any liklier to attain. However, when enemies don't drop items/stones they drop 7000 gold! 14,000-21,000 gold earned when completing Stage Dark 1-1 in 20-25 seconds is decidely more efficient than watching ad videos and blows gold income grinding in Inferno difficulty out of the water.