Mods also add 100+ hours of gameplay. There are many ways this is possible:
-Searching for mods. Searching for mods is time consuming (and pretty fun). There are over 25,000 mods for The Elder Scrolls games (Oblivion, Morrowind, Daggerfall, Arena). And that number will just increase with Skyrim.
-Playing mods. Some mods are small (fixing a bug). But some are big (remake of a city). And some are huge (new quest-line...or new lands)!
-Making mods. Probably the most time consuming thing ever, but, of course, super fun. And after you make a mod, you have the joy of playing your own stuff!
They taint the game, in my opinion. They just typically don't feel right being in there and so I don't like them.
You must have downloaded/made the wrong mod! There are tons of mods that don't make the game feel right, yes. But there are some that make them feel perfect. Don't base one mod on every other mod out there.