Orius
Unrepentant DM Supremacist
NWP's were better than nothing, but they were a lousy way of having a skill system. Especially when some skills fell under the rubric of "Thief Skills" and thus some classes got automatically on top of NWP's and would quickly go up in level.
The Players Option series (i.e. 2.5 edition) made it a point-based mechanic that let you select more NWP's and made it easier to improve them, but it was still kludgy at best.
Later 2e products started introducing some NWP's that worked more like feats, giving special combat abilities or things outside the normal range of NWP's, like rudimentary metamagic, but they were from some obscure products. (One project I did in the 2e era was to compile a complete-as-I-could-make-it NWP list, so I studied up on this).
The 3.x skill system was far better.
That about summed it up, along with what others have said. NWP weren't bad in and of themselves, but it was clunky, and the game improved on skill systems ever since. They were a flat score based directly on your ability score, and improving them was difficult, you could only add a +1 every 3 or 4 levels. And in addition, proficiency checks were 1d20 roll low as opposed to roll high like hit rolls. Another problem is that they're vague compared to 3e skills. In 3e, there are some solid guidelines on required DCs, in 2e there was some degree of confusion as to what you can do with some NWPs. Do you have to make the check for everything, or does having the NWP let you competantly perform basic tasks?
NWP"s may have been officially optional in AD&D 2e, as in the core rules said they were optional and even presented an alternate system (anybody remember Secondary Skills?), but that was a technicality. They were optional rules pretty much everybody used, as opposed to say, weapon vs. armor type rules which were optional rules almost nobody used. When 3e came out it seemed to be an edition of D&D which reflected the game as we already played it, not requiring a change of style, instead the game was changing to reflect how it was really played.
Secondary Skills were simpler, but even more vague. I did see groups that used the two rules together, even though you weren't supposed to.
One big reason that I think everyone used NWPs even though they were optional is that they formed the backbone of kits, which were the backbone of the splats. They were required if you wanted to use kits, and in some cases like al-Qadim, kits themselves were required.
I didn't know of a single AD&D 2e-playing group back then that didn't use NWP's. In fact, most groups I knew either used the expanded Skills & Powers NWP system, or a homebrew skill system that was more comprehensive.
I went with S&P NWPs. They are a kludge, but they're also to some degree a prototype of 3e's skills, and I believe they introduced synergy bonuses. I had my own collected list of NWPs from my various 2e books, and had them converted to S&P use. Certainly if I were to run 2e again, I'd use the PO system.