I just can't go back to vanilla now.
vanilla gets updates every once in a while, and we get useless stuff like villagers (nothing like pigmen, which we all wanted) and bats. Occasionally we get a good update (1.5.1).
Modded, on the other hand, has countless new things always updating. Unlimited content. The mods change the game. They make it run better, like optifine. We don't have to wait for mojang to add in stuff (like a better less laggy engine or mod API) that they never will, we can have it when we want it. It's like going to live Amish after you have a smartphone. Sure some people like simplicity, but with the mods that only add and don't change, you already have it right there.