I've heard from industry experts that Ultima Online developers gained an entire lifetime's experience in how to create an MMO, the positive and the negative. What subsequent games would divide into different categories for safety's sake (players vs. objects vs. enemies vs. items), UO lumped together in a very general way.
This simultaneously allowed for some amazing emergent behavior (such as this deer experience) and some really regrettable incidents where an admin would be, for example, manually deleting trash someone had left lying around on a path and accidentally clicked delete on a player's avatar, removing that player "permanently" from the game (I think they were able to solve it via a full state rollback, but at the cost of other players' progress to resurrect one player).