Jacob Hacker says Obamacare's lack of a mandate is no big deal. To be fair, I think the main concern wasn't so much with Obama's initial lack of a mandate. His plan, as written, might or might not leave a hard-to-guesstimate number of people without health insurance depending on various details that will, necessarily, need to be worked out by congress. It's possible that once such a plan went into effect, that it would work in some problematic way that could be fixed by a mandate, and then a mandate could be added.
The trouble is that having put out the sort of nasty anti-mandate mailers his campaign has sent around, he's now arguably closed that door, and committed himself to a policy vision that's likely to prove unworkable. I think people are probably overstating the extent to which it would be problematic to pirouette on this topic in the future, but the concern is about the future not the short-run significance of the mandate.