A Concise Summary

Re: Tax vs. Penalty - By Mario Loyola - The Corner - National Review Online: "Now, none of that makes this a tax for purposes of determining whether it may be assessed under Congress’s power to tax “for the general welfare.” In that context, “tax” is a technical term, and is highly restricted by the qualifier “for the general welfare.” 

In common usage this is a tax. For purposes of determining whether the Obama administration has kept its promise, it is a tax. But on the constitutional question, the dissent was entirely correct: This mandate is not a proper exercise of the power to tax “for the general welfare” because it is a penalty meant to enforce a mandate that cannot be sustained under the Commerce Clause or any other enumerated power.

The constitutional issue partly overlaps with the popular one: We know for sure that if the Court rules it is a tax for constitutional purposes then it is a tax for popular purposes. The administration has to face the fact that the only reason its law was upheld is that has been ruled a tax for all purposes. But outside that overlap, there is ample room to insist that this mandate is a tax, and also a penalty, and also not a tax “for the general welfare.”"

RTWT -- it's not much longer than this excerpt.