They are realistic in that the world functions as a real one, well as far as a 4+ year old game can. For example you need to hide, in real life you would head to the darkest shadows, in thief you must do the same as the darker it is the harder it is to see and be seen. In real life if you were in a room lit by burning torches and needed a shadow to hide in you would dowse the torches on the wall, in thief you do just that. In real life you run faster if you are not waving a dirty great sword about, in thief you do likewise and must put your weapons away to run at full speed. Lighting effects, gravity effects, the AI of the guards...all very realistic for such an old game, even today it remains impressive for the most part.