This will be directly contingent on who is being asked and their own impressions. Some would argue that the very reasons that could help to explain why he has done a good job could also be the very reasons why he has done a bad job. For example, the passage of health care legislation can be viewed as a monumental achievement, something that was never done in the history of this nation and one particular hurdle that had been tried and failed to be accomplished in the last 100 years. Yet, there are those who feel this is representative of a bad job. The orders to increase troop levels at Afghanistan can be seen as an example of the President recognizing the need to secure this region as critical to world security, or seen as a continuation of a failed attempt from the previous administration. I think that much which can be argued as justifying his success could also be seen as representative of his shortcomings. It is dependent on the views one holds.
I would give him credit for helping to preserve the banking system, stabilize the housing and credit markets, and passing a health care bill after it appeared it was dead (on numerous occasions). These are domestic achievements, and he has done well in terms of foreign policy too. He just signed a new nuclear arms treaty that further reduces ours and Russia's arsenals. He has vowed to close Guantanamo Prison, though he has made little progress. Troops have quietly been coming home from Iraq.
That is not to say his presidency is perfect by any means, and there are things I have been disappointed with. I think these things have been possible due to his popularity here and abroad, his realistic approach to governing, and a mostly friendly Congress.
If you think (as was stated above these are matters of opinion) Obama has done a good job, you could argue the reason he has done well is two-fold, maybe even three-fold.
1. The Senate and House both have Democrat majorities. Obama is more likely to be effective when he has legislators of like-mindedness.
2. The mainstream media is liberal. Therefore they paint a positive picture of him to the public. They also defend criticisms and choose not to report the negative things he does.
3. The history-making election has caused more people to give him the benefit of the doubt because in general America has wanted to see him be successful.
This is, of course, a matter of pure personal opinion.
I think that you can, if you believe in this, say that Obama has done a good job by pushing the health care reform bill through Congress. It had looked like the bill might be dead when Scott Brown won the election in Massacusetts last year.
Other than that, there has not been much going on. The health care bill took up most of Obama's time and energy.
You could say that the economy is starting to turn around and you could give him credit for that, too, if you want to.