Those are important points @MikeHopley and @Roni. I'm almost arriving at a conclusion that there are some actions where user does not expect a page refresh and there are some where a page refresh might not be noticed.
Take the example of creating a new thread. I think if the user is creating a thread on a separate 'Create Thread' page, then they know that they'll be redirected to some other place after the thread is created.
But then, if the user's pressing a 'Like' button, then it's expected that it gets recorded without a full page refresh. I think that's where JS should come into play.
How the comment should appear below the list of other comments is something that can be debated; and it does feel nice to have it appear without a page refresh. That's something to consider, but maybe in V3.0; not V1.0 IMHO.
I've noticed that with cloud hosted websites which have SSDs powering, page refreshes are super fast. In fact, I've developed a 'Quiz' app on WordPress that has 400-500 simultaneous users and it makes use of no AJAX! No one complained and it just works fine! I doubt I'm going to see 400 people posting on threads simultaneously anytime in near future.
Perhaps, JS could be added as the forum becomes more popular and we've thousands of users posting on daily basis. I think I can hold of JSification till then.