A lot of his more fuzzy-minded ideas -- the whole "there is no commerce and no money" thing, which DS9 finally pretty much jettisoned -- were eyeroll-inducing, but don't necessarily qualify as his worst.
His biggest weakness was probably that he was a dirty old man. And he hurt Trek the most when he let the horndog parts of him overwhelm the enlightened parts. Luckily, others managed to rein him in before Marina Sirtis ended up having to spend seven years wearing an extra breast. (Or would it have been two extras? I can't remember.)
"I don't mind the fact that race, creed, sex, ethnicity, and the disabled are protected classes, but I draw the line at stupidity." -- Jethro Elvis
: Attack otters of the ass. In submarines.