So here is some positive feedback

By nature, you're going to get a lot of negative feedback here.  So, here are a few positives, but with some caveats:

The search engine is head and shoulders above the old one.  It actually returns useful matches.  With the old, I used to advise users to avoid the "abominable" built-in search engine and use Google.  I will no longer need to do that.  Now, if we can only get users to search before posting the same issue for the 20th time.  I do see the editor notes when similar messages are found, so maybe that will help here.

I like the threading.  It's especially useful when a reply diverts from the original subject so one can see which issue another reply is addressing.  Of course, keeping users from hijacking a discussion would be better, but I don't think you could do that automatically.  It's usefulness is somewhat diminished by the poor marking of read messages, as StephenB​ has pointed out.

It's pretty easy to convert a link to a post in the old forum software to the same one in the new.  I posted how to do that in the ReadyNAS area since what I posted uses an example from there and it's bound to be  little different for each area.  It would be nice if the links here were automatically updated, but the issue would still exist for external (e.g., Reddit) ones.   But what we have is far better than the last time the forum software was changed and there was no conversion capability.  And the search engine also helps in finding the old one when you have the subject line from the link.