PROTIP: K-ON is a comedic anime. If you think it has to have a developed plot, just look at mainstream humor and say that again.
K-ON is one of my favorites, but I also like Lucky Star and Azumanga Daioh for the same reasons. One of my preferred more serious animes is Haruhi Suzumiya because of the plot.
To believe that everything that's designed to make you laugh has to have a well defined plot is, frankly, a stupid opinion. The show even has jokes based on its repetitive plot.