I would say if he plans to run as a Republican, he perhaps should start outlining his take on "hot button issues" and how he has dealt with them in words or deeds in the past.
That's a good point you bring up about how the President has to dela with so many issues. As mayor or Governor or even Senator, it's almost impossible for one guy to become an expert on everything. It'll be interesting to see how he handles the transition from dealing with mostly local issues to more broad, national ones.
Reagan handled this by focusing on a few key issues above everything. Lower taxes, less Govt, and a strong defense/anti communist policy. I think if Rudy took a similar stance and really focused on lower taxes, less spending, and good judges at home, and a strong defense and tough on jihad policy abroad, he'd do well. I think he'll get in trouble if he tries to be too much of a wonk on the issues. Only Newt can really pull that off.