Wow, that's a cool resource. Would love to see that expanded, completed and made into a sticky. That would be an incredible resource of information (and maybe a good candidate for Wikipedia - seriously).
IMO, "traditional" is a bigger category than "slip joint". Here's an excerpt from a letter I wrote to a nephew when he asked about traditional pocket knives.
STYLES - I think of pocket knifes as fitting into 4 major groups: slip-joints, locking hunters, farmer's or peasant knives and Swiss Army Knives.
The most common pocket knife is the slip-joint, which means that the blade opens and closes under pressure from a spring. Slip joints tend to be well under 4" when closed but can be a bit on the heavy side. There are a gazillion different patterns and blade styles and many of them grew out of different working traditions. (see: slip-joints.jpg) If slip-joints appeal to you, it's worth spending a bit of time looking at the different patterns to get a better idea of which style appeals to you. Both the Buck and Case web sites are good places to start.
The Buck 110 was introduced in the 1960s and attempted to provide the durability of a fixed blade knife in a folding style. These are big knives and are meant to be carried in a belt holster. The Buck 110 became hugely popular among workmen and was widely copied by many US makers such as Kabar and Schrade. The blade locks into position with a sturdy "lock-back" design which requires your thumb to unlock. Over time, smaller versions of the folding hunter became available. Those that are in the 4" range when closed become usable as pocket knifes, but there is a quite a range available from thin to thick. (see bucks.jpg)
Somewhere in between the small slip-joints and the big folding hunters are the farmer's or peasant's knives. The two most prominent examples are the French Opinel and the US "Sodbuster". (see opinel.jpg and sod-busters.jpg) Generally these are about 4" when folded. Their blades tend to be bigger than those found on most slip joints but their joint isn't as tough as brass bolstered folding hunter. They are also known for being light and arguably, they give the most cutting for the least weight of all of the traditional designs.
Lastly, there is the Swiss Army Knife (SAK). I'm including them for completeness. They are generally slip-joints but there's so many of them and the fans are so loyal, they're worth mentioning separately. (see: sak.jpg)