Standard & Poor's Rationale

Read the whole report. Much of the rationale has to do with how politics is preventing Congress from getting anything accomplished.
The irony is Obama wanted the "grand bargain" but couldn't get the republicans to agree to it. Obama wanted a $4 trillion deal with revenue and spending cuts that would've prevented the downgrade, much to the dismay of liberals. Republicans absolutely refused to go along with revenue increases.
But to the average American, who doesn't follow politics, they will blame Obama.
What does this say? It says republicans are closer to their goal of ousting Obama and shrinking government. Destroy the economy, destroy Obama.
The report starts out:

We lowered our long-term rating on the U.S. because we believe that the prolonged controversy over raising the statutory debt ceiling and the related fiscal policy debate indicate that further near-term progress containing the growth in public spending, especially on entitlements, or on reaching an agreement on raising revenues is less likely than we previously assumed and will remain a contentious and fitful process.

There's this:

Republicans and Democrats have only been able to agree to relatively modest savings on
discretionary spending while delegating to the Select Committee decisions on more comprehensive measures. It appears that for now, new revenues have dropped down on the menu of policy options.

And this:

Compared with previous projections, our revised base case scenario now assumes that the 2001 and 2003 tax cuts, due to expire by the end of 2012, remain in place. We have changed our assumption on this because the majority of Republicans in Congress continue to resist any measure that would raise revenues, a position we believe Congress reinforced by passing the act.

From the horse's mouth -- what's different is the political climate: