As with any reboot of a well loved franchise, or in this case, Thief, there is always the fear that in the name of change that certain elements of said franchise will be left out or mutated into something horrendous.
Well, fans of Thief can breathe a sigh of relief, because Adam Badke of Eidos Montreal has confirmed that you can in fact complete Thief with out killing a soul. Badke explained rather plainly that "Garrett is a Master Thief, not a killing machine!"
He mentioned that some about the game's ability allow for rather diverse ways of completing missions. "Each path comes with a set of challenges, encounters and looting opportunities", explained Badke. By experimenting with not just Garret's weapons, but also the environment, you will find different ways to accomplish your goal. He also confirmed a scale-able difficulty for both veterans of the series and those new to the franchise.
While this certainly seems like a step in the right direction, fans of the series will have to wait until Thief releases in 2014 for PC and next gen consoles before they can form a concrete opinion.