I think what people are talking about when they say that "today's music sucks" is mainstream music and music that's in the charts.
When you look at old magazines, chart listings, and what was generally very popular from the 70's and 80's, you see incredible talents like Deep Purple, Led Zeppelin, Queen, David Bowie, Depeche Mode, Gary Numan, Rainbow, Prince, Ultravox, etc.
Many of the bands and artists who made it big during the 70's and 80's literally revolutionised music and were very much deserving of all the attention they received. Those were decades when not only new genres were created, but completely new ways of creating and producing music all together were pioneered, and unlike today, they were often pioneered by artists who were in the mainstream
Whilst there are still many incredible bands and artists in many of today's genres (even pop), very few of them seem to get recognised by the mainstream compared to what I find to be standard to downright awful acts. Putting that together with how corrupt the music industry is today, and with shows like The X Factor, I can understand why people say that "today's music sucks".
Not to sound like a snooty arse, but for me, it's actually a little frustrating sometimes to see bands and artists who not only play easy-to-listen, catchy pop music, but incredibly well produced, well written, meaningful, interesting, and heartfelt easy-to-listen pop music get so much less recognition than Nicki Minaj or One direction.