well thats true. for my generation, personally, id have to say maybe basketcase by green day or a blink 182 song. thats very defining of the poppunk era, at least that stuff got me into music a lot. but i kind of mean more like the song that defines the mid-2000's. like what will be remembered. i think its mr. brightside because of the whole indie rock movement that surrounded it. it popularized indie, or helped too. i thoguht of this especially last month when i went to two parties in one weekend, and each time, even though you dont usually hear non-dance/hip hop whatever at the parties, both times, mr. brightside was played, and all the kids loved it and were into it more than anything else. its a good song.