I would say today's music is either based around
a. A sense of freedom and lackadaisical attitude towards life due to the lack of major personal conflicts and social dilemmas (besides perhaps gay rights). This is where your bands that "just don't care" and "want to party all day" come out of.
or
b. An attitude of cynicism and antagonism towards different ideals stemming from the infinite interconnections brought by the Internet. 10 years ago, would you have believed that people would be fighting over the general quality of a girls show about ponies? The Internet, as well as other social conglomerates, have exposed us to opinions we would have never faced in day to day life before. This leads to people grouping themselves with different factions and ideals while breeding an inert hatred for the opposition, even if that opposition is over something objectively petty. So, in tern, today's diss tracks aren't over different artists anymore, they're over different opinions.