He argues from a position of ignorance. He bases his builds on either houserules, complete misunderstanding of how the rules work, or complete fabrications. For instance, he believes (believed?) that a character is entitled to finding partially-charged wands of whatever spell he wants, in whatever quantity of charges he wants, whenever he wants (such as finding or buying a wand of divine power with 3 charges at level 3). Yes, most wands will be only partially charged, but wands in general are somewhat rare, are always made fully-charged, and unless you make or commission them yourself, finding wands of most spells will be difficult, at best.
He uses double-standards to an alarming degree Such as restricting his detractors to core-only when he himself ignores this stipulation whenever it pleases him (when core-only is almost never a facet of the topic at hand). He seems to be completely unable to move away from the "core-only at all times except for me" mindset.
He constantly misinterprets (purposefully, I'd wager) other peoples' comments, and argues strawmen. Constantly. Either his reading comprehension is abysmal, or he knows he can't defend his position and so moves the goalposts to something he can defend (though even then often using faulty logic in numerous and myriad ways).
He seems unable to re-evaluate his position, or learn from his mistakes. "I'm right even if I'm wrong" appears to be his worldview, and he continues arguing his points even when they're shown to be inarguably wrong.
He's also rather stupid, but that's not a mistake as much as it is an unfortunate consequence of having been dropped on his soft, spongecake-like head too many times.