According to what I see, the “Sitewide Activity” feed puts the newest postings at the top, which is good…
…except that if someone has replied to a posting, more recently, that fact isn’t taken into account in the listing order. Only the time/date of the parent posting is considered.
That means it’s less easy to spot where someone has made a new comment to a discussion you have been following.
I can imagine that there are arguments for this to work in several different ways. Maybe an option is already there, to control this (but I didn’t immediately spot it). However, I note that Facebook seems these days to work in the way that I am suggesting.