Palin's problem is that the longer she's in the limelight, the worse her ratings take a hit. That would happen at a far larger level if she won the Republican nomination and ran for president. Right now, she's a two-bit talking head on a conservative news network (or was), where people just roll their eyes at some of things she's said. However, if she's the presidential nominee for her party, every word would be scrutinized and I could see a situation where she says something so outrageous that she ultimately is done in as a candidate and loses in a landslide. Palin doesn't seem to be the type of candidate who could run a very disciplined campaign.
Which makes the idea of her ever winning the Republican nomination hard to accept. Republicans felt '12 was a winnable election and it's really the only reason Romney won the nomination. It isn't like Republicans fell in love with him - they just conceded he was their best shot. If Palin is going to run, and win the nomination, you're going to have to have a situation where Obama isn't nearly as vulnerable (or perceived) - so, how do you do that? Without doing that, Romney will still run, Palin will still be her messy self and certainly she's going to say or do something that sinks her chances. Hell, even Santorum seemed to be more disciplined than Palin ... and he still struggled with diarrhea of the mouth.