As best I recall (it seems it's not easy to find sources now) the biggest problem was with player-killing and griefing. The devs left player-character interactions relatively unrestricted in the expectation that social order would be (largely) maintained by the in-character actions of the PCs. Similar things had worked in text-based MUDs of the time. When they naturally got a Hobbesian bloodbath instead (classic MUDs have many more hours of active games-mastering per player than any MMO) they began the retreat that led to the fake-roleplaying, theme-park experience familiar from World of Warcraft etc.