Karl Smith wants to know what, specifically, "are the significant differences that you think we could actually see come to pass from a Romney Presidency versus an Obama Presidency."
This is obviously a larger question than a blog post can answer and I anticipate revisiting the subject multiple times. To be vague about it, the future is difficult to predict because congress matters a lot. A re-elected Obama administration will try very hard to implement the Affordable Care Act, but a Romney administration will find it can't just wave a magic wand and make it vanish. What occurs at that point hinges to a great extent on congressional elections and the vagaries of intra-caucus dynamics. My basic view, however, is that the Party of Romney is essentially identifical to the Party of George W Bush despite some changes in rhetorical emphasis in response to events. One noteworthy issue where this is not the case, however, is immigration. Immigration is one of the relatively rare issues on which there are major policy divides (as opposed to stylistic or tactical ones) within the Republican coalition (similar to trade or K-12 education for Democrats). Ronald Reagan, George W Bush, and John McCain all hailed from the pro-immigration faction, even as it's become increasingly clear over time that it's a minority of the overall party. Romney, in both the 2008 and 2012 primaries has run more-restrictionist-than-thou attacks against McCain, Newt Gingrich, and Rick Perry. Things sometimes happen on the campaign trail only to be forgotten later, but given that Romney's positioning on this is longstanding, in line with the views of party activists, and in line with the views of most congressional Republicans I think we should assume that a Romney administration would represent a real decisive turn of the Republican Party toward this view.
I'm not sure what the full implications of that are, but the president has considerable discretion over the precise allocation of federal law enforcement agencies' time and energy so it could be a pretty big deal. The Obama administration's desire to set a deportations record to prove a political point had a big impact on people's lives before they pivoted back to a more humane posture, and an administration sincerely dedicated to hounding as many unauthorized migrants out of the country or into jail as possible should be able to do a lot.
This would be pretty terrible economic policy in my view, but many voters seem to see immigration as a big drag on the economy so a big crackdown would be welcomed in many quarters as part of a healthy recovery policy.