I see most people complaining about the lack of facial features, yet I do not see what they could possibly add to make it more personal. Do you want to ability to add acne? scars? abnormal features?

The truth is, many people do not spend the time to customize their Mii completely. Many just use the default size of eyes, mouth, lips, nose, etc. And many features are just overused (Smirking smile?). And besides, it's not the clothing that's important for the mii, as those will change from game to game, but the personality that is expressed through the face.

there's plenty of cool ways to customize mii's. Just look at the Check Mii out channel.

I like them, they add a little extra to some games, like in Mario Kart, I love blaming the Mii that blocks the track (the 3 moving cars on the Mall track), for messing me up when I play local multiplayer.
They haven't got much to do though, theres nothing really exciting to do with them.

Miis are now what Mario used to be before he became a character separated from the user.
It is no surprise that they are loved this much.