Russ Davis, Mat Jobe, Alex Hugon from Ossian Studios and Keehwan Her from Atari have all answered community questions about Mysteries of Westgate on the official Neverwinter Nights 2 forums.
Question: How much of an impact will character class have on the story, or will the story change much if I play primarily as a caster v. sneak v. basher?
Mat Jobe, Writer (aka Nemorem): This is one area where we went all out. Your choices have a major impact on what content you see in the game - starting with your choice of allies when you first arrive in Westgate. And while other games boast "multiple endings," Mysteries of Westgate offers a completely different experience depending on your choice at one key moment in the story. We don't want to give too much away, but we're confident that players will be impressed with the amount of exclusive content that is determined by your character's choices.
Naturally, you'll also find that your choices have an impact on the myriad sidequests in Mysteries of Westgate. When we sat down to design the game, one of our guiding principles was that quests should have multiple resolutions, or at least play differently depending on the PC.
Suffice it to say, you'll need to play Mysteries of Westgate at least twice to get the full experience.