StormyFacade: Less special zones often means less loot. You get more stuff in Singleplayer, simply because you end up getting Chamber of Bone, King Leoric's Tomb, Hidden Passage (which contains the poisoned well quest), Arkaine's Valor, Warlord of Blood, Anvil of Fury, etc. The extra enemies really does make it a bit easier to keep going one more level on your first playthrough.
But in multiplayer, you can access the higher difficulty levels which drop much better loot. At least in theory, the current battle.net implementation is broken but you can still do local multiplayer.
Singleplayer isn't
meant to support the higher difficulties (the "difficulty" menu is never shown when starting a game) but there's a bug that remembers the previously used difficulty setting. So you can start a multiplayer "hell" or "nightmare" game, then back out and go into single player.
