Maybe I'm misinterpreting the system that the devs have said will be used. From my understanding, you don't lose all XP for killing things, you merely get XP points for killing things that can only be used for leveling up skills and abilities that are useful for killing things. You also get XP points for sneaking, lockpicking, dialogue solutions, etc, that can only be used to level up non combat skills and attributes. Seems a rather fair system to me, it rewards the style the player prefers with added skilss and abilities in that style. If you don't want to do the stealth, etc., it won't detract from your blood soaked advancement through the game.
Maybe I'm wrong and need to go back and re-read the posts again?