Very good points from everyone. I think the biggest con for me in GTA Online was the theme park feeling and breaking of immersion.
I hope RDR Online focuses on seamless integrations of missions/dynamic events/story/emergent gameplay within the world (at least for PvE servers / Private servers). Every time we are transferred to another loading screen or are grinding the same missions, it kills the immersion and feeling of being lost in a living world like single-player does.