I'm glad you found some of the links interesting Chris.
On my hobby site, Notaphilyc Culture “Collector's Corner” page, I often discuss what makes certain nations & series popular. Some of the reasons are logical (a shortened series like the 1993 Eastern Caribbean States flawed designs), they were just challenging to find in high grade (most older African/Asian nations with high humidity), or they prefer nations which had revolutions (renamed) like Rhodesia (& many colonies). Sometimes, there's no rhyme nor reason to what people collect (just good marketing), but generally some patterns do emerge. The following are few trends that I have noticed:
Most World collectors crave notes which have attractive designs & the larger the format (paper size) the better. I think that is why early German & Russian notes (which can be quite large) have been popular. Most World collectors (like any 1 nation-only collector) appreciate a note with good eye appeal so the higher the grade, the more desirable too. Some also seek colourful designs that reflect the culture (or history) of a nation over kings & queens. QEII nations have been popular for some folks but there are also those who pursue certain themes (technology, transportation, artists, animals) while others may collect by substrate (only paper or only polymer) in high grade. I guess it depends on what makes a nice set (for those who come from a coin background).
One thing that I found surprising was that a lot of World collectors don't care about special serial numbers (radars, Million # notes, repeaters) compared to Canadian, British, or American collectors. However, super low # (like under 10) or solid radars (8888888) will turn heads. There are a lot who value first prefixes (like AA) or no prefixes (like seen on Spanish first issued notes). It does make sense for those nations who issue many prefixes & each prefix only having 1,000,000 (or small sized) runs. And yes, small countries, with limited quantities issued, can also be popular. I hope that helps!