Overall it looks like a pretty reliable ride. We don’t seem to get many comments about difficult to diagnose problems here about the Frontier. I happened to see one on the road today (a white one) and thought it was a pretty nice looking truck. Suggest you verify that yourself using the forum search feature. I’m seeing some problem reports about an abnormally slow or rough idle on the 4-banger 2.4 engine, caused by an interaction b/t the ecm and the exhaust system. Besides that, just the normal stuff all cars seem to have, air bags, evap valves, wiring harness problems, and anything that’s automatic convenience feature, like keyless ignition, cruise control, auto door locks, moon roof, abs, blue tooth, cell phone, & ipod interface, etc.
The automatic appears to be a CVT design, so again you can use the forum search feature to see the comments here about the Nissan CVT design. I’d advise to find a version w/a standard manual transmission and as few electronic gizmos and gadgets as possible. Beyond that, make sure you ask your own mechanic to do a pre-purchase inspection, and to have some focus on the idle quality.