I'm not sure if he would make a great president.
Without a doubt he is a well respected military man but I've yet to hear of his ideas or solutions for reducing the deficit, social security, Medicare, health insurance, immigration reform, & creating new jobs.
Back in 2011 Collin Powell sated he was pro-choice. That in itself would keep him from winning the Republican party's nomination.
In today's political climate there is no room for a "Moderate Republican". Condoleezza Rice is also "mildly pro-choice".
The main challenge most military people and those with an entrepreneurial background have is they are use to delegating orders and instructions to those who signed on to obey them.
If someone disobeys them they are punished or fired.
Congress on the other hand is made up of (elected officials) that have their own self-interest and political careers to consider.
A great president is someone who is able to push through the agenda he or she promised the voters and achieving the results they promised.