I think the problem people have with nation rping is that so many people turn it into one-dimensional power grabs. It's way too common to find NRP's that involve nations headed by one character, with storylines that sound like somebody describing their playthrough of an RTS game with the cheat codes on. And you can always mark the shitty ones based on unnecessarily overpowered weapons. You could almost create a scale based on that. Count the number of aircraft carriers (or their equivalent), and then divide that by the number of named characters. If you get a whole number, you have a shitty rp, and the bigger that number the shittier the rp is. 

But that definitely is not all that NRP's can be. Like BlackBishop said, an NRP setting means everyone has a degree of control over their settings that most RP's don't allow, so that the interaction is on a macro-scale and you can tell large-scale, complete stories with quite a bit of ease. It is my favorite type by far.