Plenty of commentators, like Andrew Coyne (whose most recent article is an absolute must), are still skeptical about the NDP's ability to convert its polling numbers into a seat landslide in Québec. (See here and here.) Here are two common explanations for their skepticism, and why I think they're wrong.
Update, 6:45pm: Andrew has changed his mind from last night, and now agrees that the NDP will win over 30 seats in Québec. However, some still believe that they will only get 5 to 7 (see here and here).
1. The Bloc vote will turn out more than the NDP's.
Contrary to what many have asserted, the sovereignists are notoriously bad at turning out their vote. In almost every provincial and federal election, there is a prime à l'urne (ballot box bounce) for the federalist party/parties. The 2008 provincial election, when the PQ did unexpectedly well, was an exception and probably due to Quebecers being unenthusiastic about the incumbent government (Charest's Liberals). Such an anti-incumbency effect would clearly not hurt the NDP. While the NDP may not get a bounce because of its inferior organization, it is unlikely to be much worse than the Bloc at turnout.
2. The Bloc vote is efficiently distributed.
This is patently false. The Bloc vote was efficiently distributed in 2008 because it is quite uniform across French-speaking regions of Québec. This allowed it to win a lot of ridings by moderate margins, instead of wasting tons of votes on ultra-safe strongholds. This efficiency is predicated on the Bloc winning the Francophone vote. If the Bloc were tied with the NDP province-wide, it would still win Francophones, and Duceppe would trounce Layton seat-wise.
However, with the NDP 13% ahead of the Bloc, the Dippers are clearly also in the lead among Francophones. Both parties have quite uniformly spread votes. This means that the Bloc will lose dozens of races by 10% or less. Because the Bloc is no longer first, the same feature that made its vote very efficient in the past now makes it inefficient.
Now, obviously, there may be pockets of the province where the Bloc retreats less than elsewhere, which would make its vote less uniform and help it survive in some areas. The projection actually takes this into account through regional adjustments based on riding polls: if I applied a straight uniform swing without regional adjustments, the Bloc would be down to 9 seats.
Bottom line: If the election were yesterday, I'm confident the NDP would have won, at a bare minimum, a majority of Québec seats. Taking into account a potential recoil over the weekend, maybe that lower bound is around 30 seats for Election Day. However, while I think the NDP has probably stopped going up in Québec, I don't think that a voter recoil is much more likely than a further Bloc collapse. The best bet is that the Dippers win a majority of Québec seats, and that the Bloc loses at least half its caucus. I wouldn't even be surprised if the Bloc falls below 12 and loses official party status, though the odds of that are currently under half.