Hypothetical question here: If you could run for President this year, and regardless of your political leanings or whatnot, would you rather run against Obama or Romney?
I'd rather run against Romney, since Obama would be pretty hard to run against in my opinion.
I concur on Romney being a weaker candidate and more vulnerable to negative ads. He has lots of economic skeletons in the closet, so his much-taunted strength in experience in the private sector can be used against him.
President Obama is an incumbent with no scandals, no military or diplomatic blunders that he can't recover from, and a long spell of economic growth even if it is from a very low spot. The last President to be in that position after his first term was FDR -- who won big in 1936. His most obvious weakness is 'too much melanin' for many voters. He won despite that in 2008 and that is certain to be less relevant in 2012.
Beginning in 1900 with McKinley, 13 incumbent Presidents (counting FDR three times) have sought re-election and five have been defeated. All of those defeated incumbents had a serious weakness that President Obama does not have.