I talked about this already last week, but I wasn't really focused on Palin specifically and since Ross Douthat is keeping the discussion alive with what I think is a pretty good post, I figured I have an excuse to go back and revisit this one more time.
There are two points to make about Palin vs. Santorum, or Perry, or Pawlenty, or for that matter Romney.
The first one is that unlike Bachmann, Cain, Gingrich, Huntsman, Paul, Johnson, and probably Santorum, Sarah Palin was a plausible nominee: she had conventional credentials (just by being the most recent VP nominee), and she had positions on public policy which were firmly within the mainstream of her party.
The second is, as Douthat says, "nothing in her post-2008 career suggests an aptitude or appetite for the kind of work required to build a smooth-running (or even occasionally-misfiring) national campaign." Except it's actually worse than that. "Post-2008" isn't enough; nothing in her career after John McCain unveiled her as his running mate suggests she was likely to do the things that one needs to do to win a major party nomination.
Of course, the other perspective is that she sure did do a lot of things in 2009, 2010, and 2011 that looked a lot like running for president. Perhaps she did, and she just quit, either because she didn't enjoy doing it or because she realized she was losing (in other words, the same reasons that Pawlenty got out). It's hard to tell without more evidence; the things that sort of looked like a Sarah Palin version of running for president also look a lot like keeping one's name in the news in order to better cash in.
One more thing: would Mitt Romney really have found it hard to attack her? Maybe. But in addition to attacking his GOP opponents always from the right (as Ed Kilgore noted; sorry, just a memory, no link), Romney excelled at letting the other candidates destroy themselves. And that's something the Sage of Wasilla could be counted on to do.
Put it all together, and what you have with Sarah Palin was a plausible nominee, but also someone who did not seem likely to be a very strong candidate, and was in fact a very weak candidate during the months when she appeared to be sort-of running.