The ECB faces a unique challenge in that the Eurozone is a monetary union without a fiscal union. Because each country has autonomy over their own fiscal policy and there is no separate Eurobond, it is unclear which countries' bonds the ECB would buy to perform Quantitative Easing. In the US this is easy because of the presence of US Treasury Bonds that the Federal Reserve can readily purchase.
Also, since the Eurozone is composed of so many different economies, the main goal of the ECB is inflation control. It is extremely hard to set a blanket policy to cover the whole Eurozone when there are so many different economies in different states right now. Setting an interest rate very low or performing quantitative easing over the whole Eurozone, which is very hard because of the lack of fiscal union, would undoubtably be good policy for countries like Greece and Portugal, but would be the wrong policy for a country like Germany that currently has a strong economy.