Look at the AA timetable from Washington, D.C. (DCA), to get an idea of where they fly nonstop. If you're heading to the west coast, there's frequent connecting service at ORD and DFW (I route via DFW for more miles).
http://www.aatimetable.com/aa.pdf

If you're going to fly 50,000 miles a year, the two programs I am familiar with, AA and UA, offer 100% bonus miles once you qualify for that tier, and once you have elite status at the 25,000 mile level get things like emergency exit rows and elite check in lines. If you're going to fly 100,000 miles a year, you get additional perks, including systemwide upgrades.

You may want to spend time on the US, UA, and AA forums to get a grasp on the different programs. I know AA has a informational sticky at the top with program highlights.