Warning: spoilers follow!
The 100G Achievement "Moral Victory" is awarded to players who complete the whole game without knocking out or killing any characters. Square Enix has always maintained that it will be possible to complete the new game without killing anybody, although the confirmation in the game's Achievement list will likely be reassuring news to series fans.
Another Achievement is awarded to players who manage to reach the final chapter of the game without using the Focus ability.
The full 1000G is spread over 37 Achievements, with many awards handed out for collecting items, finishing quests, and completing the game across its difficulty modes.
For more info on Thief, GameSpot's Dan Hindes recently played the game and said it was taking the series back to its stealth roots.
Thief is scheduled to be released on February 28 for Xbox One, PlayStation 4, Xbox 360, PlayStation 3, and PC. The game's art director has said the PC version will be better than the Xbox One and PlayStation 4 versions.