Having heard this, I think it seems somewhat obvious in retrospect, but I met a smart conservative thinker last night who explained to me the conservative base's fear about John McCain in understandable terms for the first time. Basically, McCain or no McCain this still looks like a bad year for the GOP. If he wins, it's likely to be a personal win based on his persona and tarnishing Obama's persona, in which the Democrats still pick up some House and Senate seats. Next up, it's governing time. McCain's not someone who enjoys a strong personal or professional relationship with John Boehner or Mitch McConnell, and he doesn't owe any great debt to the GOP activist base. Under the circumstances, it's plausible to imagine him striking a bunch of compromises with Harry Reid and Nancy Pelosi on domestic issue in order to get a freer hand with which to conduct foreign policy.
That does seem plausible to me. On the other hand, it strikes me as an equally plausible story about Mitt Romney who had a much more substantial record of reaching compromises with a Democratic legislature. But if you put a lot of weight on purely personality-driven factors, I can see the particular fear of McCain since by all accounts he just doesn't like the Republican congressional leaders.
That said, a plausible story is just that, a plausible one. Campaign promises are a very imperfect guide to governing, but they're still one of the best guides we've got. The safest thing seems to me to assume that McCain more-or-less means what he says, and that if he wins he'll govern on the platform he just ran and won on.