This may be the most poorly conceived new feature I have ever seen on Stack Overflow.
I have lost count of the number of "opinion-based" questions I have seen posted in the new format which should clearly be classic Q&As. I posted an answer several days ago under the related Meta question (What opinion-based questions and answers have you come across from this alpha experiment that you liked or didn't like?) when I saw the first of these. In that answer I mentioned that the poster of the referenced "opinion-based" question had been confused by the UI, meant to post a regular question, and had been unable to change the type of the question.
Since then I have seen many similar questions, and many other community members have seen similar mis-categorized questions. This seems to be fundamentally related to a poor UI design, to a poorly defined concept of question categories on the part of Stack Overflow, and to a poorly communicated concept of question categories such as it is.
It isn't only neophytes who have been confused. Here is an "opinion-based" question by a long-standing member of the community, one of the top contributors in the c tag, which should have been a normal question. That user also found the UI confusing.
Worse yet, there is no way for a question poster to edit or revise their question to an appropriate type once they post and realize their mistake. And worse again, there is no mechanism for community members to collectively change the type of an "opinion-based" post, and there is no way for even moderators to adjust "opinion-based" posts when, for example, a poster requests it!
Such corrective mechanisms would be welcome in this debacle, but the new "opinion-based" question types seem to be more fundamentally flawed and I don't think that better moderation mechanisms can rescue this new feature.
The "Best Practices" category is a very bad idea that is bound to be confusing and problematic. We have always run into this confusion when an asker explicitly asks about "best practices" in a question which actually has an objective answer; many times well-meaning community members reflexively vote to close these questions as opinion-based. For example, see this recent "Best Practices" question. This question can be objectively answered (the code in the question is invalid in a way explicitly spelled out in the language standard; the posted question even cites this reference. It isn't really a matter of opinion that the best practice is to not do that and instead to do what the standard says). There is even a regular duplicate Q&A which answers this "Best Practices" question.
Aside from the lack of clarity about how questions should be categorized, I am not confident that the designers have considered well how this new feature may interact with the reputation and privilege model that Stack Overflow is built on.
The icing on the cake is that I didn't want to see any of this to begin with. I have opted out of experiments, yet I keep seeing these questions.
We have officially launched this experiment to ten percent of all users on Stack Overflow who have not opted out of experiments.... Users who prefer the traditional experience will be able to opt out of this experiment to avoid seeing the new content entirely.
For the sake of humanity and all that is good, Stack Overflow must stop this experiment now. Let us never speak of it again.