Engines,
I don't doubt that the F-35 is no longer destined to be an 'out and out' dog fighter in any configuration, but back in the day that was the objective, particularly for the biggest customer (USAF).
The ORD called for the specific performance requirements required of such high-agility fighter aircraft and to remove all doubt the requirements made repeated references to both the F/A-18C and F-16C for the KPP threshold/objectives. Indeed, I cannot recall a single parameter in the requirements set that allowed for an F-35A performance envelope that was less than currently achieved by the F-16 & F/A-18, with the exception of supersonic-dash speed.
When the ORD (JORD) for the USAF called for a +9/-3G aircraft when configured for A/A at 60% fuel, with a minimum threshold corner-speed of an F-16C, an objective corner-speed of an F/A-18C and excess mil power to sustain a 45 AOB turn above 30,000 ft or 2,500 fpm+ climb in this altitude block it was pretty clear to everyone in the program that this was not to be just a self-escorting strike fighter.
Again, I don't doubt that the end result is considerably less and that the self-escorting strike fighter description is now a good one, but this was not the objective and the new mantra regarding air-to-air capability was only adopted once the actual performance numbers fell short of the agreed threshold and objective requirements.
I'll get off my horse now, but revisionist history pulls on my chain.