As far as replayability, there are a lot of choices - some major ones - that lead to very different play experiences. Beyond that, I still play through Hordes of the Underdark for - err - research because playing through D&D games as a Wizard is a very different experience than a Monk. (I call that the Duh! moment - VD)
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As far as interactions with your companions, they're the equivalent of fellow players at a gaming table to some extent. They'll chime in, make objections and have their moments. We had a lot of fun doing those sorts of things in Knights of the Old Republic II, and it's something we definitely wanted to continue in NWN2. We also use the influence system we developed on KotOR2 because it builds in incentives to treat your companions like real people. If you want to get on their good side, you have to figure out what makes them tick. (Sadly, that didn't work very well in KOTOR 2, so let's hope they will tweak it a bit - VD)
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The system is a blend of real-time and turn-based, which makes it relatively forgiving.
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One of the challenges was adding prestige classes that advance a spellcaster's casting ability as he gains levels in that class. It took our gameplay programmers some serious work to make it so that as you swap between Wizard and let's say the Arcane Trickster - and still be progressing in spell levels you know and spells per day. What used to be a fairly static table - "Oh, I'm third level, so now I get access to second-level spells" now becomes a much more complicated "I'm third level in Arcane Trickster, but to know what spells I get, I have to consider a bunch of rules on other classes." It's not as easy as it sounds, and the result of the hard work is that divine and arcane spellcasters get access to some tasty prestige classes.​