For one, Kotor has been praised enormously for its story and world
Praise for the game does not change the fact that the main storyline is exactly the same as NWN's. The details are different. The characters are much more fully developed. There's voice acting, and cinematic cutscenes. Great. Bioware put a lot more effort into make KOTOR more presentable. The story is still copied and pasted from another game.
whereas NWN's campaign has been heavily critiqued for being little more than a fetch-and-beat-things-up-along-the-way quest.
How is KOTOR's campaign any different? You fetch pieces of the star map and beat up Sith along the way. Everything else you do is either a side quest or a mini game, and not necessary to complete the campaign.
Besides, "quest for the mighty artifact" is about the most commonly used main storyline anyway
Fair enough. However, the similarity between the main quest in NWN and KOTOR is too great for me to ignore. In both games, you must first find another object which has been broken into pieces and scattered around the world (or galaxy). I haven't played NWN for so long that I can't be sure, but I would bet that the total number of pieces you have to find in both games is the same. In both games, the quest for these pieces leads you to discover a long forgotten alien "creator race" that is responsible for what's happening in the present. In both games, this "creator race" made an object of terrible power which has been usurped by the antagonist for his own designs. The main quest in both games is more than just similar, it's identical. Only the names and locations are different.
Now that I think about it, Jade Empire's main quest has a remarkable degree of similarity as well. In Jade Empire, you quest to find pieces of the Dragon Amulet, which gets usurped by the antagonist for his own designs. All that's missing is the ancient "creator race" and their prehistoric weapon of mass destruction.