I've been gone about five weeks, and returned to find the Close Vote queue very short -- about 10% of its length when I left in mid-August. Are there fewer questions? Better questions? Is this normal summer doldrums, which I would have expected in July/Aug but not in mid-September?
I had begun to despair as the review indicator had not fallen below triple digits for some time, but now they rarely reach the double digits.
There is indeed a substantial dip in the number of questions, and especially the number of answers, during the Northern Hemisphere summer, as you can see in this SEDE query. The most dramatic dropoff, however, takes place earlier in the season. My theory is that the cycle reflects the academic year. Participation plummets starting in May after final exams are over, then picks up again in August as students return to universities. There will be a period of overlap when question volume starts to rise, but answers and reviews do not because an outsize proportion of the answerati and the reviewqueueburo and moderatariat are still lying on the beach.
If questions are improving, it isn't reflected in the average question score, which is lower in 2016 than in it was 2013 or 2014, though better than 2015.
Rather, the 2016 Community Moderator Election Results is almost certainly the biggest change, as we added two extremely active moderators while losing one who had not visited the site since October 2015. In particular, tchrist is the top reviewer of all time by a substantial margin in both the CV and LQP queues. These also happen to be the longest queues, and no doubt received outsized attention after relative neglect during the previous epoch.