I like the idea of seperating combat and non-combat entirely very much. But as others i see a potential pitfall. I am not proud of it, but i tend to do quests non-comabtly, and if i can make more xp and more loot by killing everyone after the quest, i am tempted to do so (especially if they have good loot on them). Only in recent years i strayed away from such practices in favor for a more immerse experience of playing.
What i want to say, that this should be possible without breaking the game BUT should have severe consequences in the world. For example a reputations system, so that you're known for your habbit of getting rid of your employers, or some dialog-options should could get closed, because the other NPCs don't trust you anymore, or you get more quests from evil NPCs than from good NPCs, forcing you on a route to evil.
Or you are seen as a villain and hunted by headhunters and such. Or youre getting hunted by family members wanting to take revenge. Maybe there could be connected quests throughout the game, that you don't get if you have slain the questgiver.
What i wanted to say is, that Obsidian should have an eye on power-gamers and let them play their style, but punish it in the frame of the game itself.