There are a lot of points to be addressed, so this will be a long one.
TL;DR:
Answering old questions is a good thing: the front page is important, and should not be deluged with low-value edits, but quality answers are always welcome. New questions are here. There's a 15 minute delay in place, and I often advocate for 24-48 hours as good practice. It's unlikely to increase to 24 hours, but you could try.
For some reason, people have been dredging up old questions where a answer was accepted long ago, and adding some tidbit.
This is a Good ThingTM. They're adding information to our repository of knowledge. Hopefully, questions here will not be localized in time, so information at any time can be useful to any reader. (Alternatively, they might be updating something that's time sensitive, which is laborious and not dependable, but undeniably useful!) We're not just providing answers for the OP, we're providing answers for anyone who has that question at any time.
The ancient question now pops to the top of the list since it was recently modified
Also a Good ThingTM, we want to be able to review and vote on new content.
[and] so pushes other questions where it's still possible to make a meaningful contribution off the list (off the first page really, which is basically the same thing).
Contributions to old questions are still a Good ThingTM. That said, the issue of pushing stuff off the front page is important. Low-value, high-volume edits should be discouraged, but new answers like those that Russel has been posting (containing information not presented in the other answers or an alternative approach to solving the problem) should be welcomed!
There is not a catch-all definition of a low-value edit. Use your best judgement when trying to determine whether your contribution is valuable enough to push one question off the bottom of the front page. As a few examples, I'd classify a spelling fix in the body of a post as low value, but a spelling fix in the title would affect searchability and be valuable. Adding another item to what's already a long list (like these examples..) would be low value, adding a code sample, schematic screenshot, or quote from documentation where there is none would be high value. Fixing LaTeX markdown when the question or equation is still readable is low value. If you're unsure, it's probably better to make the edit (it's still added value), but be careful or start a discussion on Meta before making a lot of them.
Questions with long-accepted answers are a waste of time. The OP has already been answered, and may never be back to see a new answer. The new information may be too late anyway, especially since it's usually just a small edit or minor wrinkle. Most of the time, the new answer or comment seems to be "Ooh! Ooh! Looky world, I done found me something I can pretend to be thmart about!". Even if you think you have a valid new angle on the topic, the accepted answer bonus has already been awarded and few people will likely read the new answer and are even less likely to upvote it. I generally just skip over questions with the green rectangle indicating a answer has already been accepted. My time is limited, so I play the odds and spend it where it's likely to be more productive.
Answering individual questions isn't the only goal here. The vast majority of our content isn't consumed by the original author of the question, or even users of this site, but by visitors from Google! We understand that your time is limited, so we want to avoid making this a support center for individuals, and instead create an easily searchable database for the public.
So, is there some way to filter out questions with answers that were accepted more than maybe 2 days ago? The reason for 2 days is that sometimes OPs jump the gun and accept answers much too early and some good discussion follows anyway.
Yes. I'm shocked that you have been posting as quickly as you do without this utility! Click the "Questions" IC in the banner, and then choose the "Newest" tab, to bring you to this page with only brand new questions. Some of these do have accepted answers, if you want only unaccepted answers, you should search for hasaccepted:0 and choose the "Newest" tab.You can also use the RSS feed to save your F5 key.
That brings up another point. I don't think it should be possible to accept a answer until some time, like maybe 24 hours, after the question is asked. Until then, you could easily get another better answer than the best so far. Twice now I saw a unanswered question only a few minutes old, so spent some time writing a detailed answer only to find that while I was typing another answer was posted and accepted. That's a big demotivator.
This is a legitimate problem, but a little off-topic for this post. On Stack Overflow, traffic is much higher, and answers are faster, so they implemented a 15 minute delay. I'd personally be in favor of a delay of 2 hours or so, and maintain that 24-48 hours is a much more appropriate time period (and advocate for it in comments), but I think you might have a good chance at getting this bumped up if you made a specific feature request for it ("Please increase the accept delay based on traffic patterns, we're not the same as SO").
