No one loves a mandate - a government requirement that we purchase something we might not want is especially unattractive. Further, individual mandates to purchase health insurance are naturally regressive, since the cost of insurance (even after subsidies) often hits those with more modest income the hardest. Opponents of health care reform are seeking state legislation and court action that could invalidate the individual mandate.
Important news out of Massachusetts suggests that our individual mandate that might not be quite "tight" enough. Blue Cross Blue Shield of Massachusetts has noted a small group of subscribers who sign up, have expensive procedures, and then give up their policies (presumably gambling that the penalty for being uninsured is less expensive than the cost of the policy once their planned medical procedure is completed.)
BCBSMA states that almost 1000 people signed up for $400 per month policies for less than 3 months, during which time they racked up over $2200 in expenses. This might overstate the case (some of these people might have had serious and terminal illnesses and disenrolled 'involuntarily.'). But still, a 'leaky' mandate isn't good enough. Making health care reform work requires that virtually everyone is in the system - and in the system through both sickness and health.