It seems to me that the main lesson is this: No matter how angry some Americans say they are, the GOP had better bring its best game if it hopes to enjoy the kind of victories many in the party are expecting.
As a National Review Campaign Spot post suggests, Republicans need to make sure that independents feel they have a reason to get to the polls. Distractions -- like third party candidates -- need to be minimized (yes, I know, as always . . .). And GOP candidates had better realize that it's not going to be a slam dunk win when their opponents are claiming conservative values.
Despite the pundits' predictions of electoral doom for Democrats on Election Day, it's far from clear that the GOP can simply count on the "national mood" to overcome deficits in money, organization, or message. The right combination of situational factors, apparently, can and will help Democrats in districts that Republicans "should" win.
But losing this election will have at least one positive outcome if it serves as an effective warning that -- whatever the polls say -- Republicans had better take nothing for granted this fall. Better to be disappointed this morning than in November.