As Kos says, "So what's going on? Our old friend, the intensity gap."
As Kos points out, if the electorate looked anything like 2008, the race would be close. But Democrats are unhappy, some because of bad news, others (according to the Daily Kos commenters) because President Obama has disappointed those he termed the "professional left." And those unhappy Democrats are not all expected to show up this year.
As a result, Republican Roy Blunt leads Democrat Robin Carnahan 45-38 (MoE 3.7), this despite the fact that Constitution party member Jerry Beck and Libertarian Johnathan Dine were listed in the poll and pulled a combined 8%. Despite two right-trending parties possibly pulling voters from Blunt, I still only show this poll giving an 18% chance Carnahan leads Blunt right now.
The way the races stand today, I think it's fairly likely that the Republicans will hold all their seats in the Senate, and gain more besides. Because, again as Kos says, "If the current malaise persists, it won't be so close." He says that about Missouri, but I think the same applies to the national Congressional situation.