I think Jim Fallows makes the crucial point:
[B]oth Reagan in 1964 and Obama in 2004 were effective because, apart from their personal skills, they added something to their party's constituency that had not been there before. Reagan began recruiting the "Reagan Democrats," starting with white Southerners. Obama tried to recruit people tired of divisive partisanship.
Sarah Palin, at least tonight, did not seem interested in bringing anyone new into the fold. A speech that was great in the convention hall. We'll see how it affects the electoral lineup.
Pat Buchanan did a helluva job firing up the base with his 1992 convention speech, and we all saw how well that served the Republican ticket that year. I don't think Palin's speech will be viewed, in hindsight, to be nearly as disastrously divisive as Buchanan's; but I also think that, come November, the pundits who are showering her with praise this a.m. (and the liberals who are shaking in their shoes) will look back on it as something less than the seminal moment they all think it is today. When the country's fighting two real wars and the economy's a mess, who has time for a culture war?