NWN1 was great mainly due to the mods, and without a doubt, specifically due to Player Resource Consortium (PRC). They added hundreds of spells, dozens of races/classes/sub-races/sub-classes, fixed all kinds of script issues, and overall almost completely merged what was available in D&D at the time into NWN1.This allowed other modders to create their own worlds with much more variety not limited as much as the official campaigns. The problem with D&D video games is that most were always limited to such a small subset of the available material and exiting rules (because there are so many additions, and a lot of open-endedness to the rules allowing the GM to create their own things, change rules to "house" rules (adding/modifying/removing certain rules), adding custom content not already described in materials, etc. PRC mod created the framework to add many of those custom things into NWN1, at least in the way of spells, races, classes).
As had been said, NWN2 was completely broken in terms of mods and support, and really did not last nearly as long as NWN1 as a result. The game had many things simply hardcoded, like the total number of classes/sub-classes/races, etc., which prevented modders from adding new ones (because to add a new one, you had to remove another one). This really showed the difference in the developer talent used to create the finished game between the first and second games (creating static sized arrays/heaps vs dynamic ones is a mistake only someone fresh out of school would make). Yeah the graphics were a little more polished, and they used a newer set of D&D rules, but the underlying code was real crap. Maybe the guys who created the engine knew what they were doing, but the ones who used that engine to create the packaged game really used some poor talent.