Without mods, Skyrim is only half as fun. That's because mods are essentially game features by the players for the players. It's arguably the best selling point of the game and it's a lot more important than you think. No, when it comes to playing Skyrim in a long enough duration, the meat lies in the game's 'moddability' or how viable it is for modding.
They aren't even the greatest assets of the said video game. The graphics, the combat, the story, the freedom of choice- those are just the cherry on top of the plain designer cake we all call The Elder Scrolls V: Skyrim.