My feeling is that the "reactions" and reddit-style "best answer" functionality should be achieved via addons.
I personally very much like reactions/ratings vs likes. I wouldn't mind that being in the core as an option as I think it's something most would consider a default thing anymore - i.e. most that are used to S.M. would be surprised not to see it or something like it. Post Ratings does the job already functionally, but presentation wise, the hover or long-press pop up is all that is missing. So this one technically is already available, just not exactly the same.
One of my dislikes about Facebook and Reddit is that other posts get buried. This is a + and a -. I absolutely hate FB in this regard. For a large group where a topic gets 200 comments and sub-comments, then I get an alert for a comment to my sub-comment, clicking the alert doesn't always (never does for me) expand the tree to get me to that reply, it takes me to the OP or some other random place. Having to "click here for more/previous comments" and search for the comment, no thanks. Not to mention the disjointed discussion that is created by that structure. It's horrible. Then people answering the question repeatedly when it's already been answered, inevitably followed by "yeah that's been answered already read the comments" and I'm gone...
Reddit is ever so slightly better in this regard where the top replies and comment trees are brought to the surface. But the issue still remains - any "branching" of discussion buries content that might be relevant to someone on a particular subject, resulting in sub-discussions that criss cross over one another, multiple people answering the same question and not seeing each others' answers...
I get wanting the option for it - some might favor it. But not me. Sounds like an addon, but it would be one that would seemingly change the default structure of threads. I can see it being done via an addon, but I would not want to see this being a default method, no way.