1. There is a recognition of the somewhat more gloomy news on the real side.
2. The difference in policy comes here:
To help support the economic recovery in a context of price stability, the Committee will keep constant the Federal Reserve's holdings of securities at their current level by reinvesting principal payments from agency debt and agency mortgage-backed securities in longer-term Treasury securities. The Committee will continue to roll over the Federal Reserve's holdings of Treasury securities as they mature.Thus, the Fed will for the time being keep the size of its balance sheet constant. They could have been more aggressive, and made moves to purchase more assets - MBS, Treasuries, or agency securities. They could have announced that they would sell off MBS and agency securities, or that they would reduce the average maturity of Treasuries on the Fed's balance sheet to something resembling what it was in the past. Relative to the other possibilities, what the Fed will actually do is a somewhat modest middle road.
The interpretation of this move is that the Fed wants to be active in responding to real events in the economy, but has some concerns about further intervention through private asset purchases. Is maintaining the size of the stock of long-maturity assets on the Fed balance sheet a good idea? There is risk, due to the mismatch in maturities on either side of the Fed's balance sheet, and I don't think anyone, including those on the FOMC, have any idea what the effects are of intervention in long-maturitity Treasuries. Now would be a great time for the Fed to sell long-maturity assets, while the prices are high, and I think this would have little or no effect on interest rates.