Out of all of the changes, this is the one that gives me the greatest pause. My first reaction is that such a facility should seldom be used, and that it should be one of the high-reputation privs rather than just about the first non-trivial priv.
I tend to think of it as being very much a "janitor" function, for use by the few that go around spending time just cleaning up. I do that, but I don't think many others do.
I think the problem being addressed is that approximately 60% of all Questions get Answered by never get Accepted. That is a quite large number, and honestly there is no way I have time by myself to Accept all those, so perhaps it does need to be something that a fair number of people can do. But still I am concerned.
I see a lot of Questions that dry out rather than getting definitively solved. People posting (good) requests for clarification and not getting any response. Now there are certainly cases where an Answer is good enough for the expressed Question, but in my experience a large number of Questions are muddled, and that even when Questions seem to be well expressed that there is very often some ambiguity in the details (though it might take someone familiar with that field to spot it.) The questions that have enough information to answer completely are usually ones that can be answered by reference to a specific FAQ section.
I suspect 500 reputation is too low to establish a history of dealing well with the kind of ambiguity that is present in those many questions.
A few days ago I was musing about this situation a bit, and I was thinking that perhaps the most appropriate would be an "expired" or "inactive" status. I would have to think more about what properties such a status would have. Not usually auto-delete, as sometimes some good discussion gets generated before the matter gets put aside.