Difficult.
First, let me give you a nice definition:
A role-playing game (RPG) is a type of game where players assume the roles of fictional characters via role-playing.
I agree with NWN being close to the original D&D, the MUDs and all the other "true" RPG's. IMHO GW isn't a classical RPG like some of us know them. On the other hand, the BBS/TelNet-RPG's are mostly gone and what is left are some strategical/statistical FPS/TPS topped with some fantasy-sauce. Just like Pinnacle said, GW is just as RP'ish as FF. Today the definition of RPG seems to be: if you can dress up like an Elf, then it's a RPG.
If we look at the definition from Wikipedia.org - and I agree with that part - a RPG has two elements: the first is that the players are making a story and the second is there can't be really a winner/loser.
1. Story making
In GW we have pre-fab stories and we can join in parts of them. We can't rewrite them or make the outcome another one. Of course we can skip them or let poor Prince Rurik get killed by Charr, but it's not possible to let him make up with King Allthatbeard (mission in Rin if I remember correctly). The players are bound and cannot make their own stories as they would please.
2. All Win
Espcially the GvG / PvP-element tells us that GW is not a real or regular RPG. Simple RPG-setting: Good guy fights bad guy over a piece of land. Good guy wins. In GW (the PvP-part) there are no Good/Bad ppl: it's full of normal players that all would like to win. Arena even gave us a ladder/ranking list to promote the competitive element in this game. The traditional RPG's are the opposite of that, they are cooperative.
I'm sure that some of my fellow-RPG-players of old think that GW can have a large RP-part. I hope they will tell me how they think to accomplish this.
(btw, sorry bout the crappy english, but too tired to look for a dictionary)