Haven't seen a game yet with a co-op mode where I didn't have this nagging suspicion "I wonder what they short changed on single player to institute co-op?" And I can actually think of an open RPG I liked (Two Worlds) where they put out two addons available for multiplayer only with 90 quests. 90! And the enemies were toughened accordingly that you really couldn't manage them solo. It sucked, almost no one used multiplayer and it was a great waste of developer time and energy. A number of folks lamented the fact the single player game got shorted so badly.
Keep single player RPGs single.
