Lucky star has no plot, as plain as that. It's purely a comedy series. The series is only driven by the most popular character of the series, Konata, even if you don't like the comedy in it, if you're an otaku (anime nerd.) you'll like her.
K-on is an anime about high school girls in a club together, yay! I am sure there won't be any extreme boredom. So in case you haven't guessed it's not that interesting, but they're all in a band together, thanks to the Light and Music Club. The series starts off with only four members of the band, and later on at the end of the first season we get a new character, sadly she is not interesting either. Now don't get me wrong, the music their band makes is good, but the characters are, well, dull. The show follows them in their normal life practicing, making new songs, and wait for this one - buying food! But never the less it's still cute, I guess.