The main problem with stack-orient programming languages is that they are conceptually quite difficult for humans to understand. The advantage is that they are very easy for computers to evaluate and generate.
Which is why it was chosen for PostScript. It wasn't that long ago that printers only had a very small amount of memory and very under-powered CPUs. So giving them programs in a language that was easy to interpret and also easy to generate, you didn't require a super-computer inside your printer to evaluate.
Also keep in mind that many languages today, while not stack-oriented themselves, actually get compiled down to stack-oriented bytecode (think Java and .NET). Again, the reason being that stack-oriented processing is very easy for a computer to reason about.
Best Answer
As far as I know, Self is the original language that invented the "class-free" paradigm based on prototypes. It already existed (in an experimental stage) in the 1980s and pushes Smalltalk's elegant usage of the prototype pattern to the extreme, such that classes are completely eliminated.
It influenced all the other "class-free" OO languages I know of: