Normal, Nightmare, and even perhaps Hell difficulties, this is true. This breaks down at Inferno. They tested Inferno difficulty for lvl 61 mobs which appear in act 1. Indeed, you can beat act 1 pretty handily with only gear you find. This becomes way harder for the beyond the first, act 2 in particular, in which mobs of lvl 62-64 appear. If I remember correctly, they did NOT test for lvl 62-64 mobs.