Michael Barone's first "lesson of the day" is that Americans are fed up with legislators who sit on the Appropriations Committee, because three losing incumbents were on it. That's right, Barone thinks that the majority of Republicans and Democrats follow and give a shit about who's on the Appropriations Committee, or even know what the Appropriations Committee is, and that this is one of their main issues as voters. Nonwithstanding that Specter's primary challenge was founded on the fact that he switched parties and was not a true Democrat, Bennett's challenge was basically a strategic "we can find someone slightly more conservative in this blood-red state of Utah," and Mollohan lost the Democratic nomination for his congressional seat in West Virginia because West Virginia an "ethnic white" conservative state, and also a very Democratic state which throws back to the Dixiecrat era in the South, and the Democratic primary is essentially the general election where Black Big City Obama's guy just lost to a Good Country Folk.
There isn't some easy connection to explain what happened today on both sides of the aisle, when factors are very different from Republicans to Democrats. Republicans are still trying to prove that Bush was so unpopular not because he was conservative but because he wasn't conservative enough (yikes!), so they've been punishing their establishment in favor of nontraditional candidates in what is an essentially a pipe dream that politicians who are dispositionally vitriolic, irrational and quick-tempered will lead them to political success. Rand Paul is as colorful a character as his father, so Republicans went for him as their senate nominee in Kentucky, over mainstream conservative Trey Grayson, who Senate Majority Leader Mitch McConnell had endorsed. This also explains uberconservative JD Hayworth's pending primary challenge of John McCain in Arizona, GOP senator Bennett's loss in Utah and Marco Rubio forcing Charlie Crist to run as an Independent in Florida. (If you are a Democrat, you ought to be cheering these Republican efforts on because even if they are short-term successful, they will marginalize the party later on.)
Democrats, on the other hand, are probably just sticking their finger in the wind and realizing that incumbents are unpopular, so might as well defeat them in the primaries so they have at least a chance of winning a general election, and if they fail then it's no sweat off their back. They aren't running from Obama, who, with approval ratings of about 50, are one of the highest for any U.S. figure in a time of high unemployment and abounding economic trouble. Sitting Arkansas senator Blanche Lincoln is dead in the water whether she wins her primary or not. Her primary opponent Bill Halter will face an uphill battle in the general election if he wins the primary, but polls show him doing a little better against the Republican opponent than Lincoln, so Democrats abandoned Lincoln for Halter and now there will be a run-off contest because neither one got 50%. It was sweet revenge for Democratic progressives, who are still sore after Lincoln made herself a major stumbling block for healthcare reform in order to appear moderate, but Democrats in Arkansas wouldn't have voted for Halter if he didn't seem to have a better chance of winning than Lincoln. In Pennsylvania, Sestak beat former-Republican Arlen Specter because he was doing a little better in polls and voters are keen on anti-incumbent attitudes.
If you want an overarching narrative to explain all of this, it's simply that Democrats won on the Change narrative in 2008, but now that narrative is over so both parties are looking for a new one, and their candidates can only guess at what the electorate will decide on so always risk being dumped.