Q:

How do we improve the quality of content produced on Puzzling.SE?

I feel like it's time to open this discussion in its most general form. Countless recent meta discussions (okay, fine, they're countable) keep coming back to a fundamental issue with this site: our content quality is rapidly declining. Not all of these discussions have strictly been about the quality of content on the site, but the discussions have invariably lead to concerns about the quality and direction of the site.

In private beta, our site was very different. We generated comparably complex, expert questions that generated theoretical, interesting, and intriguing answers. We still have those, but we've recently been flooded by "solve this puzzle" questions. 
In the days of yore, we discussed whether specific puzzles were on-topic at all, and came to the general conclusion that specific puzzles are on-topic, though they need answers with complete explanations. We've seen answers nowadays that are simply "Is it >! [answer] ?", and these are terrible answers. But this isn't our only problem. We later asked whether riddles are on-topic, and at this time, we seemed to be heading toward 'no,' or at least 'yes, with limitations.' 
Later, Jon Ericson, a community manager, asked the question of whether chestnut puzzles are on-topic here, and this entire question and answer are worth reading.  Other questions worth reading:

Are specific coded message puzzles on-topic in Puzzling?
Why are questions off-topic if they invite answers which are not demonstrably correct, or are otherwise speculative? (this is the basis for our only existing custom close reason)
"The oracle was lying!", "Just stab them instead!" and other... 'creatively' unhelpful answers (alright, I posted this one, but it hasn't received responses yet)

In short, our stance on puzzles as questions and pretty much every other quality issue can be summarized in one word: "conflicted."

I think it's safe to say that we were not the least bit prepared to handle the sudden and terrifying influx of traffic. There wasn't strong enough consensus about what is and isn't on-topic, what constitutes good answers and questions, and what voting means to us. I doubt our traffic will be spiking fifteen-fold like it did over the past few weeks anytime soon, and this is our chance to work through some of these problems.
However, we absolutely need to address the quality of questions and answers on our site. We need to promote high quality content, and turn away low quality content. We want to attract experts, and experts are being driven away.

Let's keep our precedent firmly in mind, then take it with a grain of salt. Precedent is great, but we have issues to address, and our site has changed in many ways since much of this precedent was set. Let's focus on developing solutions that are directly appropriate to the quality concerns we have right now. We can make any decisions we need to about what's on topic and what isn't, what is good quality and what isn't, but whatever we decide here needs to drastically improve site quality. If that means disallowing questions-as-puzzles, then let's do it. If that means aggressively deleting low-quality answers, let's do it. But we do need to do something, so let's figure out what that something is.
What are the first steps to raising the quality of questions and answers on our site? What specific actions do we need to take to push us in the right direction? We need to figure out what's most important, and how we hold each other accountable for upholding our new policies. What needs to happen?

A:

We must define what is on-topic in a clear unambiguous fashion. I haven't been around all that long, however every discussion I've seen about what's on-topic has been very ambiguous and I'm still not remotely clear about what the story is. Indeed, the consensus in meta is often internally inconsistent. I argue that any definition of quality will need the guidance of what's on-topic. 
We are talking about 2 disparate sites. You are talking about something like CSTHEORY for puzzles ($Site1$). And others are talking a PPCG with riddles as challenges ($Site2$) (the answer that fits all the clues best (this is another matter) wins).
You say experts are leaving. The one you mentioned was a lover of $Site1$. Well, I retort that experts are coming... for $Site2$ (Mathematicians, Physicists, and Computer Scientists). This is really a debate about which of the 2 we want to be. Frankly, I want $Site2$ - however the union of the 2 would be fine for me - but I'm seeing that many people are not ok with this union; they want $Site1$ and only $Site1$. It seems clear that there is no reason for them to be leaving on this basis anyway - there are filters available for the 'riddle' tag. Regardless, the community must decide where it stands on this - not a couple of people whining about what they want. I see many people entering the discussion who don't even have a single question or answer on the site and wanting to dictate what the community should be. But, I apologise as this is besides the point.
There are people for both, some will like one and some the other. I say we should be both, some just want one. This is what the topic is about.
I, for one, will leave if $Site2$ is locked out from on-topicness. I will give all my rep to avigrail and rand al'thor over a set of bounties and then the riddlers will forever reign supreme!!!
All in all, to answer your question clearly. We need to decide which of the 2 we are, and define on-topicness in a clear unambiguous way and make it clear to new comers (as I was not long ago) so that they can enforce it and decide if they want to participate.
Let's talk about e.g. riddles being on-topic. The issue of quality is complex and requires its own specific question (i.e. when is a riddle question good quality?) - and let us please avoid "I'm on a diet therefore you can't have a cookie".
ADDENDUM: I'm not saying that all challenges that are riddles or riddle-like are gold. Nor am I saying that anything goes and I'm definitely not saying that PPCG just accepts any challenge. They (and we!) must make sure the challenges are interesting. (I love spaghetti as much as the next person but too much will kill you)

A:

As one step in this process, we should be aggressively deleting answers that give absolutely no explanation. While normally I would say "downvote them," this has become such a rampant problem that I'd be totally in favor of deleting them.
Expert answers to expert questions. Not guesses to expert questions. 
Here's a short list of answers which have absolutely no explanation associated with them (a lot of these are on one or two questions, but that's just because I'm lazy): One, two, three, four, five, six, seven, eight, nine, ten, eleven,  (deleted) twelve, (deleted) thirteen, (deleted) fourteen.
Okay, so they are answers technically, but they are also very, very low quality. Each of these is a one-line answer that gives absolutely no explanation as to why it's right, and should (at least) be downvoted. That makes them simply guesses.
But because this problem is so rampant, deletion may be the preferred route. We'd want to leave a comment, letting users know why their answer was deleted, so they don't just suddenly come back to confusion. Something like, possibly:

Hello! Unfortunately, as this answer does not provide an explanation as justification, it has been removed. If you can edit it to include a complete explanation as to why your answer is correct, feel free to flag it and it may be undeleted. Thank you!

We have other categories of quality issues, too, and this isn't meant to address those. However, I (at least) find these answers terrible, and am inclined to say they should be outright removed. If this idea gets actionable support, and you see such an answer, flag it.

