I like the changes to the close reasons and the mechanism itself a lot and I think it is a significant improvement and clarification over the old system. But there is one aspect that I think has the potential to do more harm than good, and that is the new ability to use custom close reasons:
- Closers can enter a free-form reason ("Your question appears to be about 'Cat Grooming', which is off-topic for Stack Overflow.") Free-form reasons will be presented as a comments, but the close dialogue will refer the reader to the comments for more info
- Free-form reasons picked by closers will be available to subsequent close-voters on that question as one of the selections from the list
I don't think it is a good idea to let any user with the ability to close define a custom close reason. While this specific implementation avoids some issues by delegating the actual custom close reason to comments, I still think that custom close reasons have more dangers than benefits. They might make the opinion of a few close voters appear more official than it actually is, though the implementation reduces this danger somewhat by putting the custom close reason into comments.
But I also found that the predefined close reasons are an important tool to provide guidance and keep the close votes in check. We don't close questions just because we don't like them, but because they violate some clear rules that we have set up previously. With the new ability for each communtity to set up their own off-topic close reasons, we have everything we need to define our own rules. A close reason should be something the community discusses and debates, and something that should be decided with clear support of the community. It should not be something a handful of users can just put into a text field.
Any user can still add a comment with specific that are not explained in the close reason, but without the official backing that free-form close reasons would provide.
I don't think we need free-form close reasons, forcing any new close reason to go through the process of being defined as a site-specific close reason is a good thing in my opinion. So I propose to remove the option for free-form close reasons entirely.