I've felt a same general idea that our network was having issues, but I wanted more concrete data, so I ran some numbers through the data explorer, and created some pretty charts.
Q & A with variation (lower is better)

Questions opened and closed per month

Average score per question as a percentage

Q&A Ratio

Unanswered Questions by Month (%)

Please feel free to zoom in to get a better idea of what's going on. Basically, each data point shows the number of net points that were accrued for questions and answers since 2012.
As you can see, we had our last strong peak was January 2017, followed by decreasing bounces back. It seems at this point we've flatlined around 1,000 points a month this year, down from closer to 2,000 for the past four or five years.
I know we recently got the "new contributor" marker to help us identify new users, but we need to do more. We need to encourage everyone to do better questions. The answer quality hasn't fallen from what I can tell, it's just that there's less questions to go around, so fewer answer scores to go around.
However, the overall trend still reflects that older questions are apparently higher quality than newer ones. We should try to find a way to get better numbers. We're still answering a decent number of questions (the question to answer ratio is good), but we need to focus on helping better questions.
The open-to-close question ratio is interesting. I thought that might show a lot more closures, but really, we've been holding steady at about 120 closes a month.
More information (and substantial edits).
We're still closing at the same rate we have for a few years now, but our unanswered ratio has gone up considerably in the past year, and voting is down. Answers still get upvoted pretty consistently, but the questions do not (we're currently at about 1 question upvote per 2 answer upvotes, shown in the Q:A ratio graphic).
Questions are being abandoned more frequently, with up to 30% of questions from any given month going unanswered; this trend started almost exactly a year ago, while almost all historical months are under 20%. As cropredy suggests, this might be related to new subject matter for which we have few experts. It may be useful for us to learn or find people who know about these subjects and help us out.
Also, we need to find a way to deal with the more graphical questions. For example, people want help with a Flow or Process Builder, but we have no way to help them without a long dialogue just to figure out what they're even trying to do, specific settings, etc. We might need some Q&A-style canonical questions that we can use to at least link to new contributors so we can skip the dialogue and get meaningful results.
In addition, I suggest that we start more aggressively cleaning up tags on questions. Let's make sure that new contributors are tagging their questions correctly so we can get experts looking at those questions. Incorrectly tagged questions might not get seen by experts with the new watch/ignore tag feature.
And, when applicable, we should remind people, especially new contributors, that it's okay to accept a best answer, it's okay to upvote, etc. And I think more people need to be encouraged to vote overall, or, if they choose not to vote, we should encourage them to suggest edits, add comments, and otherwise generally help improve question quality.
We need at least a few people to look at the questions at a deeper level, start cataloging commonly unanswered questions, and break it down into a few different types: duplicates we can't close, difficult subject matters, commonly mistagged questions, etc. The more of an effort we put in to this, the better we can make our community.
I think at this point we're going to have to go through a bit of a rough patch to clean things up, but I also feel it's worth it. With some better resources, maybe some "quick training" videos, like has been previously suggested, a library of great canonical questions, perhaps a set of common comment templates we can all share (I know Adrian's got a bunch of 'em), we can improve our network.
At this point, I think we have enough evidence that the network needs help, and we need to find specific ways we can help the network and implement changes to provide higher quality questions and more consistent solutions for people that need them. We can organize details in further meta questions, so please don't hesitate to post if you have any ideas.