An explanation that struck me recently is that "rationality" memes have infected a much broader audience such that now people don't all have to come to the LW watering hole to get their fix of sanity. Tim Ferriss references Kahneman in his podcast. "Heuristics and biases" is no longer a fringe thing that nobody is interested in. It's also somewhat less weird to be Singulatarian, now that you can point to IBM's Watson and self-driving cars and Kurzweil working at Google, whereas five years ago you just sounded crazy.
So, it's not necessarily that LW is getting worse in any absolute sense, it's just that the world is catching up. LW is just less special.
There are various people who feel that Lesswrong degraded in the last year. In the same timeframe more and more discussions moved into the open thread model and open threads became weekly instead of monthly.
I suggest to counteract that trend by opening all discussions per default on Discussion instead of opening them in weekly open threads. Having the topics in discussion makes it easy to browse the list of topics and choose the headline that are of interest, even if the thread got opened two weeks ago.