They knew McCain wasn't exciting to their base, so they added Palin specifically to help with that. But she quickly became a net negative for that ticket, still exciting their base (most of whom would have voted anyway, excited or not), but turning off way more moderates (who either didn't vote or voted Obama).

You're right that Trump seemed to expand on a lot of the Palin-style rhetoric and it seems to have worked for him. But that overlooks the fact that Trump was seen as the more moderate candidate of that race, on policy, largely because he explicitly ran against his party's longstanding priorities on Social Security and Medicare. Trump wasn't running as a partisan. He was just running as an asshole. And Americans like assholes.

Palin was very exciting to her base but poison to the general electorate. I just wish the Dem base could see that such a dynamic can exist, and that certain policies that seem great to us and work as candy in the primary can then kill us in the general election. Biden, as terrible a campaigner as he is, seems to be the only one (other than no-shot candidates like Hickenlooper, Bennet, and Inslee) watching out for this. The primary is not the prize.