For me the primary issue has been the words, the lexicon, the cadence; it's flat. Seems like writers have learned to write and learned what's right, they've been writing and not reading. There's not much exploration of the form, it's all so grammatically correct. Maybe it's the fault of the editors or typesetters, if those still exist. It's boring, either way.
Secondly it's the themes. Struggle and oppression have always been plot drivers, but it seems like a vast majority of works published in the last decade are just slight variations on "oppressed person struggles to exist within the System, then finds a way to overcome; The End." The Hero's Journey highly compressed, lossily.
Luckily there are hundreds of years and countless intriguing novels written before all this, more than I could ever hope to read in my lifetime. Maybe this is what it is like to age: to recognize truly that nothing is new under the sun; so, go forth by going back, seek that which was once new in its time.