The Canadian Press article about McKenna's withdrawal lists Brison, Ignatieff, Stronach and Tobin as the lead remaining candidates. I'll be surprised if any of those names make it very far, except, perhaps, Ignatieff. (Although, how could Ignatieff ever win an election when he hasn't lived in Canada as an adult?)
Instead, there are other candidates that will rise to the forefront. Kinsella mentions Allan Rock. Wells reminds us of Dryden. I've already mentioned that I join several other bloggers in liking Stephane Dion. Calgary Grit is intrigued by the rumours of a Gerard Kennedy candidacy. And then there is Bob Rae. Others with potential may rise.
With McKenna and Manley out, the chances of a quick rebound to power are diminished. If the Liberals choose a less-well-known individual as their leader, the public will need time to get to know them before handing over the reins of power (unless Harper fails miserably). I'd say the expected duration of Stephen Harper's Prime Ministership just went up. The upside for the Liberals is that they can truly rebuild with fresh people and fresh ideas.