Some good points from Jeffrey Simpson about issues that are coming up but which no political party really wants to talk about for fear of having to make some sort of decision that might turn voters off. He gives a list of them including the impact of an aging population, the future of health care, the lack of an energy policy, what foreign policy looks like if it's not just me-tooing the Americans, productivity/competitiveness, immigration policy, and the lack of progress on aboriginal issues.
There has to be others too - I'd suggest racism. While just about everyone in politics is willing to condemn blatant acts of racism everyone is much less willing and/or able to discuss the existence and results of the more systemic kind of racism that sees black and aboriginal kids do more poorly than others.
I'd have to say this one is the big thing-we-don't-want-to-touch specifically on the left:
These include words such as “productivity” and “competitiveness” that countries such as Sweden and Finland and Denmark worry about and act upon all the time, as opposed to Canada, where the mere mention of the words, let alone what might be done about them, strikes fear and terror into political hearts.