The big thing about MMOs is that they (try to) create communities, and being part of a community is a surefire way to make people stay.
Give too much freedom for a single player and the community as a whole falters. Give them enough to do without groups, but leave the "good stuff" as something exclusive to groups and don't give dungeon finders or similar auto-grouping tools. I say this as someone who spent way, way too much time playing WoW like a single-player game
