1. Stabilized the American economy. When the Republicans first voted against the stimulus, the stock market lost 1000 points and they had to make a 180 degree flip flop turn and re-voted. Obama's leadership during the first two years was crucial, and we saw the stock market climb into the mid 12000's (before the debt ceiling mess) and unemployment begin to improve, albeit slowly. The economy continues to improve slowly, but it's remarkable that this is after several plunges caused by Republican intransigence. Just look at the votes on debt and jobs corresponding with stock market data.
2. Financial reform. I used to work for the bank and I was appalled at some of the practices. When I confronted management with it, they told me they (the corporation) couldn't take the higher road without incurring huge financial loss, and it would take an act of congress to make change happen so that all financial institutions played by the same rules and would thus negate the loss. Obama promised and delivered on the reforms I personally understood to be crucial to reform in the banking industry.
3. He has defeated Al-Qeada. Authorizing more targeted missile strikes in his first year than Bush did in eight, Obama has been relentless in pursuing terrorists, and with only one leader of Al-Qeada left, he might just be able to dust off that old mission accomplished banner before November.