Many of Romney's strengths (listed in his Profile) are the same as in 2008. The Republican establishment favors Romney, even though they have fears about whether conservatives will accept him. He is still likely to have a well-funded and well-organized campaign. This will be Romney's last real chance to become president, so he'll be even more willing to spend his own money than in 2008.
Romney has one new strength: He has branded himself the "economics" candidate. If the economy remains in focus during the primary, it will be to his advantage. During the 2008 run, Romney struggled to brand himself, floundering about for a central theme to his campaign. He finally settled on the economy, but it was too late in the game. This time, Romney is ready.
Then there is the issue of Romney's liberal record from his term as governor of Massachusetts. Last time, Romney was hammered for flipping from moderate to conservative. Romney's opponents were able to put the "flip-flopper" label on Romney very effectively. This time, Romney has at least been consistent for an additional few years. He will not be quite as much of a "flip-flopper," but his sincerity will still be suspect.
Romney has an additional problem: The Tea Party strongly dislikes and distrusts him. No one can say exactly how much influence the Tea Party will have in the primary; it will likely depend on whether Tea Partiers rally behind a single candidate.
Romney's biggest problem, of course, is Romneycare--the Obamacare-like legislation Romney enacted in Massachusetts. It was a problem last time, but it will overshadow everything else this time. It is possible for Romney to employ a strategy that will minimize the Romneycare problem, but he has not yet discovered it. The strategy is discussed in Romney's Profile.
Finally, Romney has the chance to use what he has learned about the early primaries from his last run. During the 2008 race, Romney split his resources, attempting to win both Iowa and New Hampshire and coast to victory. Romney seriously underestimated how weak he was in Iowa, wasting resources that would have been better spent in New Hampshire. His second-place finishes in both states did not impress voters. Romney did not anticipate a long, protracted campaign. This time around, expect Romney to put all of his focus on winning New Hampshire. Early polls suggest he should do well there.
In summary, Romney has some new strengths but also some new weaknesses. But he has the ability to mitigate some of his weaknesses--should his campaign be conducted wisely. Elephant Watcher calculates Romney has a 10% chance of winning the nomination.