If you'll bear with me, there are two different answers: the official answer and the realistic answer.
The official answer
There is no difference between the two questions: the web developer question should be closed. We originally had a lock reason for this, but this was removed and we were told to close as not constructive any question that started to amass too many answers. The web developer question was closed, but was later reopened by the community.
Since it seems pointless to keep closing a question that can easily be reversed—even though that's the official policy—we haven't bothered to re-close it.
The realistic answer
The value in the Stack Exchange engine is that, for every question, there's going to be an answer that floats to the top. That answer has been vetted by the community (through voting) or by the asker (though accepting) as being the correct answer. So, when a visitor comes along in the future and has the same problem, they can just read the top answer and have their problem solved.
And so the major difference between the maintainability question and the web developer question is that in the web developer question, there is one answer that compiled all the knowledge from every other answer, effectively providing a definitive and comprehensive answer. You don't need to read any of the other answers because the first one covers them all.
That is, despite the question essentially being a poll, one answer is so damn good we overlook the question's other flaws. It's a rarity: an exception to the rule. One out of every hundred or so similar questions has the community actually take the time to meticulously maintain such an answer.
But 99 times out of 100, we just get a poll, where every answer just lists one small piece of the puzzle. If you wanted to even begin to understand the full answer, you have to read all the answers. But since none of the answers actually fully solve the problem, all the things we cherish on Stack Exchange—community vetting and voting—are meaningless.
Everybody's answer is equally valid and invalid at the same time because there's no way to judge what the correct answer would be: if you have an opinion about the subject, you technically provided an answer. Not really ideal at all, and not really what Stack Exchange is about.
That's what the maintainability question turned into. Despite deferring early flags and putting up a post notice to help keep the question on point, we still got junky answers we had to delete and nobody stepped up to provide a comprehensive answer. So when 3 members of the community voted to close and we got a flag pointing out what it turned into, we closed it.
Thankfully, there's a really easy workaround for getting questions like this to be on-topic and avoid the trappings of being labeled a poll and amassing a bunch of answers that don't quite cover everything: just provide a specific problem the answers can be tested against:
I'm working on project A, where I'm trying to do B. I want to make sure that when I pass it off to my client or the next developer, that all my bases are covered with respect to C. How can I make sure my project A is maintainable for the next guy?
Straightforward, to the point, and more importantly, testable. Someone can look at the situation, read the answers, and judge whether any specific answer actually helps the situation.
However, while this would've been helpful to prevent the state it's in now, what we have now is a ton of answers that don't really fully answer the question. We can fix that, too.
When a question automatically gets converted to community wiki (after getting 15 answers), it's supposed to be a not-so-subtle hint from the engine to the community that existing answers should be improved and/or consolidated: once a question goes community wiki, nobody gets any reputation anymore and post ownership gets heavily downplayed.
So while none of the 18 answers fully solve the problem, if we could create one comprehensive answer from all of them, then we'd be cooking with gas. If someone would like to do this, feel free to flag any answers that have been incorporated into the comprehensive answer and we'll delete them.