I'll give it a try, since no answers have been posted so far.
The problem: Improve popular questions and answers over time
Subproblem 1: Finding questions and answers worth improving
Popular questions are those that get most traffic. Let's say, the threshold is 50% of the total views. These are few questions because the distribution is highly skewed:
(distribution of questions by the number of views, source)
This formulation of the problem is preferable to asking to write good answers right away because it's not always evident that the question is hot, so it's better to save efforts until it becomes evident.
The hotness becomes evident when questions continue receiving views over time.
But users can't even track the questions that get hundreds of views each month over years. The "Hottest Questions Today" section covers only short-term peaks in activity (generally, the SE interface encourages the fastest gun behavior, even though these daily hits soon get forgotten). The dynamics of views is unavailable on data.stackexchange.com either. It has "total views", but not "views on week/month/year t".
Subproblem 2: Late answers and improvements are not encouraged
The reason is that late voting (week plus after the question was posted) is rare, otherwise the votes wouldn't have exponential distribution on a logarithmic scale:
Users are roughly divided into two groups: (1) active community who browse the front page, give answers, vote, and moderate; (2) passive readers who google questions, read, and close the page—the bounce rate is 52%. The active community monitor changes. But since changes are rare, late voting by community members is rare.
I found no mechanism that encourages late revisions or at least indicates their necessity.
Solution: Encourage passive users to vote
Passive users come from search engines and they don't vote, as seen from the data in the question above.
Putting the problem of registration aside, there're 330K of users who can vote but never did:
How to encourage these eligible users to vote?
I suggest two ways:
- Reminders while the user is looking for answers
- Delayed reminders, when the user has time to vote
I can think of traditional reminders, such as:
- Popups (done by SE devs before to encourage upvoting for questions)
- Emails (SE does send notification emails)
- Special pages for listings (SE has a page with "hot daily questions," for example).
All these forms are practiced already.
It's harder to make such reminders friendly. The best way is to reduce the reminders to the necessary minimum, so that the reminders concern only:
- Questions with daily traffic above average (quality and, hence, feedback are important)
- Answers that captured the user's attention.
- As indicated by mouse interactions (clicks, selections, hovers) with specific answers. This says which answer the user had taken but didn't vote for.
- Limited number of questions per time period.
Asking for three votes per week by email would yield millions of feedbacks on important questions.
These upvotes, in turn, encourage authors to improve existing and write late answers.