Here is my $.02 take on a comparison of the two games:
EASE OF INSTALLATION: Diablo-2 (D-2) wins. D-2 was insert CDs, wait, and play. NwN took a little effort, because I had to find a drive with 1.9 Gbytes free.
SOUND: D-2 wins this one. NwN has more sounds, but I had to adjust my sound card to allow for a less fascicle interface.
EASE OF PLAY: D-2 wins this easily (intentional pun). NwN is based on D&D with all the capabilities, but including the corresponding complexity of D&D. You have to spend some time reading and playing with NwN before you can take advantage of all the options.
INTERFACE: NwN by a nose. D-2 is simpler, push and kill, but NwN lets you do A LOT (order of magnitude) more stuff. There are around 20 hotkeys in D-2 if you count things like "I" for inventory, and there are around 50 potential hotkeys in NwN (it allows you to use SHIFT and CTRL with the Function keys for a total of 36 there alone). D-2 has the short-cut bar, and NwN has a short-cut bar at the bottom along with a short-cut panel to the side. NwN also lets you right click on characters and objects to bring up potential actions. Easy to use and handy.
GRAPHICS: NwN by a hair. D-2 has more detail (polygons per object) and effects (e.g., varied shadows, two point perspective) while NwN lets you rotate 360 degrees, vary camera angles, zoom, and put the player's view in different modes (i.e., following, above, headlights). So, six of one, half a dozen of another. (I'm inclined to give NwN the edge on this one as more than once in playing D-2 I fished around with the Alt-key depressed, trying to pick-up something that fell on the ground in an awkward location; so, I can see benefits of the moving camera for sure.) The difference in graphics between "the coral, diadem of the leopard" and the "the jade diadem of the lion" are not so important to me.
CUT-SCENES: D-2 wins. Very impressive cut-scenes in D-2. NwN tend to be more atmospheric, but less effective.
PATCHES: Toss-down. Both games have big patches. A pox on both for that!
SAVING: NwN wins hands down. You can save NwN at any point. You can stop at way points in D-2, but things tend to reset when you do. For a guy who plays occasionally and doesn't have hours at a stretch to play, having to re-do everything from scratch every time you opened the game in the Cathedral of Chaos was maddening at best.
ROLE-PLAYING: NwN by a lot. The quests in D-2 were very linear, and the NPCs were more fixtures than anything else to provide particular services, e.g., identify magic items, sell potions, repair armor. (How many times did Mr. Cain tell you about the Horadric (sp?) cube? "That's quite a treasure you have there ..." "Yeah, but I've just met Diablo and want some advice!") NwN has NPCs that react based on your character's previous actions. You have a chance to vary your dialogue with them, and you can bribe, hire, etc. the NPCs. Being able to download or create and play whole new levels with your friends is just amazing, and the capability to GM with the NwN system is very impressive.
FLEXIBILITY: NwN wins. Your on-line and personal characters are interchangeable; so, if you want to take your 15th level wizard that just completed the single player version into the dungeon of doom on-line there is that capability. GMs can select whether they allow off-line characters in their setting or not. The ability for users in NwN to (relatively) easily create their own adventures is great! (Certainly, there is a potential down-side too. Because the creation process in D-2 is close hold, the levels release tend to be play tested. The NwN scenarios don't have a rigorous review process; so, you can end up with some real lame or unbalanced jewels.)
MULTI-PLAYER: Could go either way. Played in D-2 Battle.net, but never did it with a group of friends -- I'm told that "that is the magic" in D-2. Was underwhelmed by my battle.net experience. NwN has settings that can prevent character on character engagements, and the settings are visible from the site where you chose to enter a game, but I can imagine that in the wrong scenarios, NwN could end up being like Ultima On-line (not so good). In good scenarios, NwN could be amazing. NwN has a number of built-in "hot key" comments like "follow me" or "I need healing" that would tend to speed up play.
OVERALL: No disrespect to D-2, but got to give it to NwN.