First up is the financing issue,
Funding: This may be the stickiest difference between the two bills. The House version raises a substantial chunk of the needed revenue by imposing a 5.4 percent surtax on incomes over $500,000, or $1 million for couples. Meanwhile, the Senate bill raises revenue through an excise tax on "Cadillac plans." Many wonks believe the tax will put downward pressure on spending by making these expensive plans less appealing, but there's still concern that the tax could hit many middle-class people. The conference could just split the difference, implementing versions of both taxes.
As I've said for some time when folks like Ezra Klein and Matt Yglesias have been upset with union opposition to the Senate funding scheme, if you spread the pain around you probably lessen the rhetoric emanating from labor. Funding healthcare reform solely on the backs of middle-class Americans is bad policy and bad politics. Spreading the funding around is certainly more palatable.