Initially when Quora was launched it seemed quite similar to Stack Overflow, but as the site has progressed it seems to me that it's actually quite divergent in terms of the core focus of the platform.
Stack Overflow is all about getting great answers to questions. Everything, from the reputation system, answer sorting, moderation, etc is designed so that the questions that are asked are on topic and objective, and that the best answers are situated right underneath. Not surprisingly, this focus was borne out of frustration with Experts Exchange playing tricks and hiding the answer to the question on the bottom of the page.
Quora, on the other hand, is all about social connection and discussion. It does have a ranking system, but kinds of questions and answers that are typically found on the site are much more subjective and are designed to encourage many people to weigh in and provide different points of view. Quora also places higher importance on the person's real name and employer than a Stack Overflow-like score for on-site contributions.
Overall, I think the title of Scoble's post ("Is Quora the biggest blogging innovation in 10 years?") sums up the difference quite clearly - Quora clearly has more in common with blogging than it does with straight-up Q&A like Stack Overflow.
