Since 1900, only one Republican became president by defeating an incumbent. That was Ronald Reagan, in 1980. He had these four traits that might have helped him:
1. He was conservative.
2. He was a governor.
3. He sought the nomination, in a previous election.
4. He was from the West.
These Republicans, who might run, have at least three of those four traits: Sarah Palin, Dirk Kempthorne, and Mitt Romney. I hope that all of them will run, and I hope that Tim Pawlenty and Buddy Roemer will run. I don’t know which of those five I’d support. I’ll decide in the end of June. I hope that at least four of them will continue campaigning until the convention. If that happens, no one will get the majority of the delegates before the convention. The convention will be more suspenseful and exciting, causing more people to watch and hear the great republican ideas. That happened in 1980, and Reagan won about 40 states.
One with those traits won. How many people since 1900 had those traits and didn't win? I'm guessing a whole lot more than one.
Those four traits don't necessarily make a Ronald Reagan. Unlike the others you have mentioned, Reagan had an incredibly likable personality which made it hard for even his enemies to personally dislike him and which made a lot of people who would have otherwise voted against him, vote for him. Reagan was a popular governor of a giant state which could provide a large percentage of the necessary electoral votes for victory. Reagan was the undisputed leader of the conservative movement. The dynamics of 1980 are a lot different than the dynamics of 2012 which means that those traits wouldn't guarantee a victory (although personally, I could easily vote for both Palin and Kempthorne).
Making points 2 and 4 fit Mitt Romney is a real stretch. I know very few conservatives who would claim him. Michigan and Massachusetts (his two strongest claims to a home state) are hardly western states.