I'll give you two possible answers to think about for each.
1) Health Care Reform - There are many people who love and many people who hate this legislation, but the fact remains that it is the most comprehensive attempt at health care reform since the mid 1960s, and the only serious attempt during that time to address the growing problem of the uninsured.
2) The War against al-Qaeda - No, Obama did not kill Osama bin Laden himself, but he does deserve some of the credit for refocusing our mission in Afghanistan to hunt for him, and for allocating the necessary resources towards that goal and giving a gutsy order when we weren't positive he was even there.
1) Gitmo - Obama promised on the first day of his administration to close Guantanamo Bay prison camp. Not only has that not happened in two and a half years, but it doesn't appear that it will happen by the end of his first term.
2) The deficit - It is now at the highest level in US history, and our debt is a serious threat to future economic stability and growth. While other Presidents share the blame, Obama hasn't done enough to forge a budget compromise that reduces the debt and puts us on solid footing once again.