its all down to opinion, and i can't change yours :) noone can

K-ON wasn't meant to be a serious show, im quite sure of that. it was an easy-to-watch slice of life that didn't need much effort to watch. that's why i liked it, plus i wasn't bothered by fanservice (though im not into foreheads xD)

i myself can't understand why it became so popular to the extent it has (crazy obsessed fans etc.), for the reasons above that you gave, but whatcha gonna do? =D there'll always be obsessed otaku nuts who devote their life to a show and buy everything there is merchandise wise.

it wasn't a groundbreaking show, i don't think that was it's intention.

the only thing about K-ON that bothered me was there wasn't enough music.