Postfix notation isn't really related to stack programming, you could just as easily read the programs right-to-left and have prefix notation. Something like K/Q but with stacks. Anyways, stack based languages are a real distinct category, just like OO or FP or array-based languages or whatever. If you look at forth code for example, especially written by Moore, it's some... pretty crazy shit. I do agree that conditionals are pretty annoying to read using stack based languages, but they are used a lot less often due to the implicit "branching" a stack context provides.