The public option isn't really the main deal breaker any longer.
Lately it's been more about the taxing of so-called "Cadillac" insurance plans and the abortion language. The reason that the "Cadillac plan tax is now such a sticking point is this.
The Senate Healthcare bill was passed on December 24 with it's primary funding a tax on high-price insurance plans, which would have included not only CEO type plans but also insurance plans that Labor unions had negotiated for, often ceding wage increases in return for the security of comprehensive healthcare for their families. This funding mechanism was already a compromise by Senate liberals who preferred to raise funding by a tax increase on wages over $250,000. It was a deal breaker for organized labor so President Obama subsequently met with union leaders and brokered a deal that would: increase the minimum qualifier for family plans but not single plans; increase by five years the grace period before the tax would kick in for union members and excempt add-ons like dental and optometric care. There was no mention of how to redeem the 40% shortfall in funding.
   These changes would have been ammended to the Senate bill by the House before sending it back to the Senate. Now, because of the loss of the of their filibuster-proof majority, the only way the bill could be passed would be for the House to pass the Senate bill unammended, in fact word-for-word as it came out of the Senate on Christmas Eve. This would not even have to be rushed through before Brown is seated because it would never have to go back to the Senate before Obama signed it into law. So the deal Obama netotiated with labor would be negated and labor betrayed. If you don't know why this matters, it's for the same reason Republicans and many conservatise Dems as well, won't go against their corporate sponsors.