I played vanilla for quite a long time before installing mods. I thought "mods, what's that; meh... I don't need them." Now I have 100 + mods! 
The first mod I installed was an alternate start mod because always starting at Helgen was getting boring. I've also played around somewhat with the creation kit and made my own houses and stuff.
Most of my currently installed mods are for the looks, and mods that add more armors, weapons, hairs, eyes, warpaints etcetera because I like to have a lot to choose from. But I want them to fit in with the world and not be overpowered.
I have ASIS for more enemy spawns and enemies actually use potions etc... "Immersive creatures" which also adds more - and more varied - enemies... and one mod that adds more dragons. "Immersive patrols" which adds more travellers to the roads, and more stormcloaks and Imperials.
I use UFO so I can have more than 1 follower at once... Convenient horses (before I installed that, I almost never used horses, and that's one of the few mods that I in fact consider necessary)... and other mods that change stuff that I thought weren't done right... Such as being able to change your spouse's clothes... Or to know which book is a skill book and what it tecahes, so my characters don't waste leveling points on skills they aren't interested in.