The only real problem with the NWP system was that it was implemented haphazardly. The basic concept is sound, and to me the 4e skill system is basically identical to a cleaned up NWP system.
The basic idea behind NWP is that you are either skilled or unskilled, and that your chance of success or failure doesn't depend much on level but on your ability score (and perhaps on a degree of specialization or focus). Early editions do this with static difficulties and 4e does this with ever increasing bonuses largely matched by the expectation of ever increasing target numbers (or difficulties), but its basically the same idea of 'fixed math'.
To me then, the merits of the NWP system are basically the same as the merits of 4e system - fixed math, ease of play, and high focus on character concept. To a certain extent, because the old system allows for increasing breadth of skill through the course of play (as opposed to just depth, which 3e tends to encourage) I like it better than the 4e system and see merits of it over the 3e system (though not enough to adopt it into my homebrew).
The problem with NWP's in practice is that some fit this core concept well, while others departed from it. One of my favorite NWP's was in fact a proto-feat that took some element of the game that D&D had never treated as a skill and gave you a bonus in it. Some NWP's were quite narrow and some quite broad. Others were just not that well thought out mechanically or had unique systems all to themselves. In this way, they were alot like the 2e concept of 'kits' - good basic idea but frequently very poor implementation.
But you could clean up the system as part of a general house cleaning of the older editions and by and large I think it would work as well as any skill system in any system.