They do have differences between them
I would agree with many about Romney being Obama lite when it comes to big government and the role of government. Clearly there is some differences between the two that have big impacts on what happens to the country moving forward. There certainly are more reasons but here are three reasons you could choose not to vote for Obama.
I will start with the Supreme Court, When you consider the two justices that Obama appointed in the first term that more bad choices will follow. Sonia Sotomayor and Elena Kagan are clear liberals whose view of the Constitution, it's meaning and it's interpretation. You have four justices in their 70's and Obama could be appointing one or two in another term. We could be stuck with a liberal/progressive leaning court for a very long time if Obama could get one or more passed the Senate. The difference between who Romney would select and Obama would be very different.
The second one is even simpler than the firs, it's called Obamacare. Romney has stated from the moment go that repealing Obama's healthcare law is something he would do. So if you dislike the law and wish to see it repealed then voting for Romney makes that possible. The law Romney passed also have a mandate for health insurance, something that many dislike including myself. The State have different rules than the federal government, they have the right to make laws that are different. They experiment with ideas, ones that work other States will sometimes follow suit and when it doesn't other States stay away from bad ideas.
The third one is Fast and Furious, The scandal that still hasn't totally been explained by Obama, Holder, the DOJ and ATF. The problem isn't just idea of running guns without tracking them, telling the Mexican government about the program or anyone else for that matter. Do you remember Obama running in 2008 about bringing a different, having a more transparent government in Washington. The cover up over this program, intimidating witnesses, the unwillingness to come clean about what happened.
I'm sure everyone can find many more reasons, I think too many have the belief that Romney and Obama are the same. They are far from the same, however they may still both expand government in different ways. The above three reasons along with how he's handled the economy could make anybody believe that Obama doesn't deserve a second term.