The basic answer is that, downticket, yesterday simply wasn't as big a "change" election as anticipated. Take Alaska, which didn't even see fit to trade in two of the most loathed Capitol Hill denizens in decades!
If Democrats had underperformed because people were dissatisfied with Congress, which is one theory, then you would have seen two always-shaky Democratic seats, John Barrow's and Jim Marshall's in Georgia, up for grabs last night. Even in 2006, these two Democrats, then already incumbents, only squeaked by. But Barrow and Marshall coasted to victory yesterday. A number of incumbents who'd been running behind in polls, like Pennsylvania's Paul Kanjorski and Wisconsin's Steve Kagen, won in the end. And most of last night's rematches, usually a good chance for a party to flip a seat, saw the incumbent significantly strengthen his hold on his district, from Vern Buchanan in Florida to Jean Schmidt in Ohio to Baron Hill in Indiana to John Yarmuth in Kentucky to Carol Shea-Porter in New Hampshire.
I suppose it's disappointing that Democrats didn't have as huge a tidal wave as we'd hoped.
On the other hand: It does suggest that Americans didn't simply vote for "change" last night, from the bottom of the ticket to the top. They voted for Obama.