Marc outlines them:
Democrats assume that the immigration debate will open the curtain on the Tea Party movement; health care will be child's play compared to the tantrums over the prospect of earned legalization and other measures. The overlap between the Tea Partiers and ethnocentric immigration restrictionists is huge, and even many Republicans worry that the embedded nativism in the movement, whether or not it is also racialized (as a proxy for being against Obama and his ilk) will come to the fore in a way that once again diminishes the fervor of right-leaning independents and energizes Hispanics.
But forget immigration even: the next two big presidential initiatives, domestically, at least, will be -- or should be -- easy political victories: reauthorizing but reforming the education law, and financial services reform.