I don't usually post on forums, but I think this time I will,
There are three major reasons why I think Morrowind and Oblivion where great games, I'm not sure if others will think the same, so I'd like to make my feelings known.
The first is the loneliness that can be felt in both games, wandering about the wastes, making sure that your characters got back to a city before nightfall, none of which is necessary for the gameplay but just enhanced the RPG nature of the game.
The second is having a home; I absolutely loved 'Rosethorn Manor' in Oblivion.
Finally the exploration, not part of the main quest – just finding things and places for no other reason that just to explore. In Fallout 3 vault 22 – finding that was very interesting, the contrast with the surrounding areas.
So if there are any developers reading this, if you want my top score, get the loneliness, exploration and expand on the owning of homes and allow building camps too.