I don't have Skyrim for PC, but I have played Knights of the Old Republic and I modded the crap out of my game. I remember I was going crazy, overdoing the mods and basically installing anything I could find. I was about to install a mod that gave Bastila a bigger butt or something and I paused and went, "What the hell am I doing?!"
I imagine it must be similar for Skyrim PC players.
As Pete says, you have to be careful. Even the most harmless looking mod can have unintended effects. The earlier in your mod-loading life that you discover this, the more cautious you are.
I have:
Armor and clothing mods to give more variety;
wet and cold and cloaks of winter, so that npcs don't wander around in blizzards in nothing but fur bikinis;
nude mods because the undies are appalling and most underwear mods feature decidedly 21st century looking thongs or lacy things (people didn't wear underwear in the Middle Ages, so I don't need them to in Skyrim either) and also because I find it very odd when people freak out at my nakedness and I'm not, actually, naked;
mods that make characters and players more attractive;
a couple of house mods (one that adds an enchanting table to Breezehome, so that I don't have to murder Farengar for talking NON-STOP when I'm trying very hard to maximize the usefulness of a few enchanting potions);
the unofficial patches;
a mod that unlocks more "respectful" guard dialogue;
and a mod that makes most unarmored npcs run into a building when a dragon or vampire attacks (rather than pummelling it with their little fists or iron dagger).