Airports subsidised from local/national taxation.
Avgas 1/4 of the cost.
Land 1/4 (say) of the UK cost.
Widespread instrument approaches.
No need for tower ATC for instrument approaches.
Area ATC can provide IAP service without invoicing the airport for it.
Much more accessible IR, resulting in a much higher level of GA activity.
Much higher activity resulting in higher asset utilisation.
Greater public tolerance for aircraft.
No Class A (i.e. VFR OK) everywhere below 18,000ft.
Class D transit implicit with a 2-way radio contact.
Class C/B similarly easy.
Most sizeable towns have a runway, and fuel.
Large unified market (no Customs)
No PPR.
No landing fees.
Few parking fees.
Large unified airspace (most Americans never have to go "foreign").
Decent comprehensive free weather services.
Pilot briefings available free over the phone.
Easy IFR flight plans (none of the Eurocontrol crap, hours spent working out a 200nm route in Europe).
Comprehensive enroute radar service.
etc