What makes these games so much fun is the well designed story. It is coherent, epic, and involved. For a story created to support a series of video games, it is far more than you expect. A good story alone, however, won't make a successful video game. If the game is boring or frustrating to play, you will eventually give up and move along to something else.
I didn't play the first two Elder Scrolls games, coming to the series in sort of mid-stride with Elder Scrolls III: Morrowind. The fourth and fifth installments, Oblivion and Skyrim, continued the fascination. In my experience, all Elder Scrolls games contain three distinct elements:
1. Your character can become anything. Yes, you choose a race and a class. In your average game, that choice sets the boundaries of what your character can learn and do. In the Elder Scrolls games, your character can follow whatever path you choose. Do you want to have a big, burly Orc that wears heavy armor but likes casting healing spells? No problem. Of course, there is a cost to the choices you make. That Orc is going to run out of spell casting power (called magicka) much faster than would a Breton (a different race). The freedom to decide your own path is an Elder Scrolls trademark.
2. You can explore a vast world. Another thing that sets Elder Scrolls games apart from the pack is the sheer size and accessibility of the world. There are no invisible walls preventing you from seeing what's behind that building or over that next hill. Exploration is permitted and even encouraged. You should be forever scavenging, looking in every sack, chest, pot, crate, and barrel that you find, seeking materials for crafting or supplies to sustain your in your adventuring.
3. You can effect permanent change. If you steal from the citizens of a town, and get caught, you will be branded a thief. Your reputation will spread ahead of you and you may not enjoy a very warm welcome in the next town you enter. If you assault a guard, you will be killed or arrested and, assuming you're not dead, you either pay a fine or spend some time in an unpleasant dungeon. If you build something, go away, and come back, that thing you built is still there. Likewise, if you destroy something or kill someone. If you effect change in the world, that change is permanent.
The first speaks to a good game design, the second to a lot of hard work by developers, and the third to playing in a single player game. You are the only player in the world, the rest of the computer generated inhabitants exist only to serve as the backdrop for your adventure, and there is no need to be concerned with how what you do might affect someone else.
The Multiplayer Quandary
When I heard they were planning an online version of the Elder Scrolls, I immediately dismissed it as impossible to pull off. You simply couldn't make a world as vast and changeable as what we'd grown used to in the single player games and let more than one person play it at once. Or so I thought, anyway.