I have mixed thoughts on the feed.
I don't think a social feed exactly like the example proposed is desirable as posts are all different lengths (particularly mine lol). So, it would just create a huge amount of scrolling on phones in particular. It would also just make the site too busy, overwhelming people with too much information. It would just make everything look cluttered to have a social media feed like the example displayed.
Having said that, I do see value in having 2 or 3 line snippets of each topic shown (similar to an email snippet), because sometimes the title is not enough information for you to see if the topic would be of interest to you. So, a 2-3 line snippet makes sense for the new forum - a full length social media style feed does not.
The second point - about the reply nesting - which I assume would be similar in concept to Reddit, does make sense in that people can divert a discussion into it's own thread without derailing the main topic. However, I do dislike the inability to making all pluses on reddit expand by default, so I can read the nested threads as well as the fact that often it ends up showing the answer on a new tab really irritates me. So, conceptually I like the idea that Reddit has, but their design implementation is really poor and user unfriendly.
But the problem with nested threads is that generally I want to read them all as sometimes interesting things are said, even if I am not strictly interested in the diverted topic itself per se.
So, yes I see the value in both options, but how it is implemented would either make or break the site.