While there are a number of problems, a protracted development process, unrealistic (and constantly changing) requirements, “cost as an independent variable”, and the need for jointness, but this article makes the point that when we look at the performance issues of the airframe, they are almost entirely a function of the STOVL requirements foisted on the airframe by the US Marine Corps:

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But the chorus of praise is wrong. The F-35 Joint Strike Fighter — a do-it-all strike jet being designed by Lockheed Martin to evade enemy radars, bomb ground targets and shoot down rival fighters — is as troubled as ever. Any recent tidbits of apparent good news can’t alter a fundamental flaw in the plane’s design with roots going back decades.

Owing to heavy design compromises foisted on the plane mostly by the Marine Corps, the F-35 is an inferior combatant, seriously outclassed by even older Russian and Chinese jets that can fly faster and farther and maneuver better. In a fast-moving aerial battle, the JSF “is a dog … overweight and underpowered,” according to Winslow Wheeler, director of the Straus Military Reform Project at the Project on Government Oversight in Washington, D.C.

And future enemy planes, designed strictly with air combat in mind, could prove even deadlier to the compromised JSF.

It doesn’t really matter how smoothly Lockheed and the government’s work on the new warplane proceeds. Even the best-manufactured JSF is a second-rate fighter where it actually matters — in the air, in life-or-death combat against a determined foe. And that could mean a death sentence for American pilots required to fly the vulnerable F-35.

‘Can’t turn, can’t climb, can’t run’

The F-35’s inferiority became glaringly obvious five years ago in a computer simulation run by John Stillion and Harold Scott Perdue, two analysts at RAND, a think tank in Santa Monica, California. Founded in 1948, RAND maintains close ties to the Air Force. The air arm provides classified data, and in return RAND games out possible war scenarios for government planners.

In Stillion and Perdue’s August 2008 war simulation, a massive Chinese air and naval force bore down on Beijing’s longtime rival Taiwan amid rising tensions in the western Pacific. A sudden Chinese missile barrage wiped out the tiny, outdated Taiwanese air force, leaving American jet fighters based in Japan and Guam to do battle with Beijing’s own planes and, hopefully, forestall a bloody invasion.

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To add insult to strategic injury, one of the most modern Chinese prototype warplanes might actually be an illicit near-copy of the F-35 — albeit a more intelligent copy that wisely omits the most compromising aspects of the U.S. plane. It’s possible that in some future war, America’s JSFs could be shot down by faster, deadlier, Chinese-made JSF clones.
The Chinese J-31 appears to be based on the F-35. Via Chinese Internet
The F-35 that could have been

At least twice since 2007 Chinese hackers have stolen data on the F-35 from the developers’ poorly-guarded computer servers, potentially including detailed design specifications. Some of the Internet thieves “appear to be tied to the Chinese government and military,” Defense Secretary Chuck Hagel claimed.

The September 2012 debut of China’s latest jet fighter prototype, the J-31, seemed to confirm Hagel’s accusation. The new Chinese plane, built by the Shenyang Aircraft Corporation, bears an uncanny external resemblance to the F-35: same twin tail fins, same chiseled nose, same wing shape. “It certainly looks like the Chinese got their hands on some [F-35] airframe data,” said Richard Aboulafia, a vice president at the Teal Group, an arms industry consultancy in Virginia.

But the J-31 lacks many of the features that were included in the F-35 “mainly or entirely because of STOVL,” according to Aviation Week writer and fighter expert Bill Sweetman.

Namely, the J-31 does not have a lift fan or even a space for a lift fan. The omission apparently allowed Chinese engineers to optimize the new plane for speed, acceleration, maneuverability and flying range — and to add good pilot visibility and a second rearward engine — instead of having to build the plane around a pretty much useless vertical-takeoff capability that slows it down, limits it to one motor and blocks the pilot’s view.

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Jet design like any engineering practice requires disciplined choices. The JSF is the embodiment of ambivalence — a reflection of the government and Lockheed’s inability to say that some things could not or should not be done. “It’s not clear with the F-35 that we had a strong sense of what the top priority was — trying to satisfy the Marines, the Navy or the Air Force,” said Air Force Lt. Col. Dan Ward, an expert in weapons acquisition who has been critical of complex, expensive development efforts.

By contrast, the Chinese J-31 does not appear compromised at all. Surrounded by rivals with powerful air forces — namely India, Russia, Japan and U.S. Pacific Command — and with no grudge-holding Marine Corps to hijack fighter design, it would make sense that China prioritized the air-combat prowess of its new jet over any historical score-settling.

That apparently apolitical approach to (admittedly illicit) warplane design appears to have paid dividends for the Shenyang-made jet. “With no lift fan bay to worry about, the designers have been able to install long weapon bays on the centerline,” Sweetman wrote. The centerline bay helps keep the J-31 skinny and therefore likely fast and maneuverable — in any event, faster and more maneuverable than the F-35, which in a decade’s time could be pretty much the only new U.S. jet the Chinese air force might face in battle.

If Stillion and Perdue’s simulation ever comes true and the U.S. goes to war with China in the air, F-35s dragged down by their lift fans could be knocked out of the sky by Chinese-made F-35 clones that are faster and more maneuverable, because they never had lift fans.

A couple of notes here.

First, much of the baggage of the lift fan occurs even with the non-lift fan variants, the tubby fuselage, the poor aft vision, and an engine that has a thrust to weight ratio of engines from the 1970s.

I would also note that the 20 year long development process did not help.