I’ll defer to Nate on the details but just wanted to add a couple of general thoughts.
My quick answer is that you can’t learn much from primary elections. They can be important in their effects–both directly on the composition of Congress and indirectly in how they can affect behavior of congressmembers who might be scared of being challenged in future primaries–but I don’t see them as very informative indicators of the general election vote. Primaries are inherently unpredictable and are generally decided by completely different factors, and from completely different electorates, than those that decide general elections.
The PA special election is a bit different since it’s a Dem vs. a Rep, but it’s also an n of 1, and it’s an election now rather than in November. Nate makes a convincing case that it’s evidence in favor of the Democrats, even if not by much.
I don't agree with Nate if he really thinks the Tuesday primary elections provide evidence in favor of the Democrats. Critz ran against his own party on all the major current issues. He could do that because he wasn't the incumbent. Thus the Democrats (at least the ones who voted) in PA CD-12 sent a strong message to Obama and the national, "we don't like what you are doing, stop it." Running against their own party is not going to work for Democratic incumbents in the November election. They will most likely pick up the PA CD-12 seat, but many others are in danger. The common thread that ran through the primaries: the voters are not happy with the current Congress. How can this be good for the Democrats?
I note that Nate keeps using the word "moderate" when he should be saying "liberal." And he says "progressive" when he should really say "far left." The Democrats have trended left over the past several decades dragging the center with them. A "moderate Republican" is now really a liberal Republican.
The U.S. has generally been a center-right nation except for a few places like New York City (where I was born and raised). Center-right rejects the European model of governance in favor of a central government with limited and enumerated powers. In my opinion, it's the European model that's getting rejected by the voters and the Tuesday primaries confirm that.
Zarkov:
Ideology makes a difference, and I expect you're right that Critz's conservatism (relative to other potential Democratic nominees) got him some votes. Still, it the voters in that district really wanted to have elected a Republican, they could've done so. They chose not to, and that provides us with some information.
Andrew:
I've heard (but not verified) that Critz was actually more conservative than his Republican opponent. If that's true, then the PA CD-12 voters might be more concerned with ideology than party label. On the other hand, let's also not forget that the Democrats enjoy a 2 to 1 advantage in party registration in PA CD-12. So we should not be surprised that given two conservative candidates, they would pick the Democrat. Why not?