I've always thought that there were too many options there especially as some are always going to mean different things to different people.
The only important one and the only one I use is Latest which, coupled with marking all threads read at the end of each visit means that I can go straight to my All Discussions list and immediately see what's new in the sections I'm interested in which, compared with Lithium, is simple and just what I need.
From having a look at the options I don't use, I can't honestly see anything that I'll ever use. For example, why would anyone want to look at the most unpopular threads? Beats me.
So, what do they mean...
Latest appears to list threads ordered by those with the most recent activity first.
Top seems to order by total post count.
Newest orders by the thread starting time.
Oldest is just the opposite of Newest.
Popular (why does it take so long to populate?) seems to order by total views.
Unpopular (also takes forever to populate) and is the opposite of Popular.
I'd suggest dumping Oldest and Unpopular.
Rename Top as 'Most replies'.
Rename Popular as 'Most Views'.
That'll simplify things and I'll just carry on with Latest! 😀