A character specialized in necromancy (Spell Focus and Greater Spell Focus) can do some serious damage with spells like Wail of the Banshee. Gnomes make great illusionists because they already have Spell Focus: Illusion, freeing up a feat to spend on something like Toughness or armor proficiency feats for use with Still Spell.

Lone-wolf Wizards and Sorcerers have a better time in NWN than they do with a group in a real D&D game.
Out of spells? If you are playing D&D with a gaming group, you will have to sit out the entire session if resting is nowhere in sight (a constent annoyance in a Living Greyhawk game). If you are playing NWN, one hit of the R button and you are good to go again.
Traps and locks? In D&D, you'll need either ranks in the cross-class skills of Disable Device and Open Locks or get yourself a Rogue. In NWN, Find Traps disables all traps except the quest ones (read: Isle of the Maker) in a huge radius, and Knock does the same for all locks.
One hitpoint? In D&D, no amount of bandages will help you without resting without magical assistance. In NWN, use a healing kit and you are back to full instantly.
Multiple enemies? Isaac's Greater Missile Swarm and Greater Sanctuary.