GameSpot:

Mask of the Betrayer is a much more personal story than the original NWN2. The entirety of the story is about your survival and what you choose to do to survive. So, instead of it being about saving the Forgotten Realms--again--it comes down to what you will do to save yourself.

The story is more of a continuation of your character then it is of the NWN2 storyline. We tie up many loose ends from NWN2, but the things you are doing aren't directly related to the events of the original campaign.
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As far as the companions being more fully developed goes, Kaelyn, who is completely optional, has more than twice as many lines of dialogue as the most developed companion from NWN2.
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We've taken a somewhat different approach in Mask of the Betrayer in that we've applied that idea [alternative paths] to the whole game, but not differentiated it so clearly. The core focus of what is happening to you in the game can really be affected by how you choose to deal with it. That in turn changes how the game reacts to you as you continue playing, and ultimately how the game ends. But at any time you can choose to alter what course you're on. This isn't so clearly a "good versus evil" decision, though alignment is involved in some ways.
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In a lot of ways, I feel like players are not just getting an expansion pack, but a whole new game in the world of NWN2.

GameShark:

I think we've made some major improvements to the Influence system. First, now your companions (and you, perhaps) will gain game play benefits for having high influence. Second, if Influence goes low enough, the companions will behave more realistically and actually leave you. Also, we've spent a lot of energy on polishing how Influence works with the companions. For example, increases and losses will feel more natural and appropriate to the characters' personalities and goals. Except for the very beginning of the game, none of the companions are required and players who don't like them can adventure alone if they wish.​