Although Brian Topp was seen as the establishment candidate and was hyped as the early favourite, we shouldn't be surprised that Mulcair came out on top. Since his Outremont by election win in 2007, everyone has assumed Mulcair would succeed Layton. Although he hemmed and hawed at the start of this leadership race, he was the best politician in the field, he ran a good campaign, and avoided the pratfalls that usually plague frontrunners. As a result, he was able to grow his support on each subsequent ballot - more so than Topp in fact.
As a Liberal, it's easy to scoff after the fact and say Mulcair is beatable. Many Liberals will point to his flaws, especially after watching a very unimpressive victory speech. However, I wrote before the vote that Mulcair was the most dangerous candidate for the Liberals and that remains the case. The NDP have squarely aimed their sights on the centre of the political spectrum, and they have a polished politician to lead them there.