There are a number of differences (attribution, the way the stream shows you edit diffs from your team members, the way content such as code and media is handled, the way email integration works, etc).

But fundamentally it is a difference of design intent. Hackpad is not a replacement for a word processor. It is not designed for cosmetically designing papers or reports. Hackpad has been designed from the ground up for collaboration on ideas and for living documents (which evolve over time). It's in use in a number of leading tech companies (airbnb, stripe, upworthy) as well as on the open web.