His severe social anxiety, holo addiction stemming from an obsession with imaginary characters, and self conscious behavior and attitudes extending into a perception of inadequacy; in addition to his fear of transporters, which was cute when Bones did it because it was sort of a joke, but with Barclay it was a crippling disability at least until he overcame it, all contributed to the hindrance of his performance on numerous occasions.
Now granted, knowing a thing or two personally about disability, I'd never be one to say what one should or shouldn't be able to do, but I also know where that 'don't discriminate against me because of my disability' ends and where I have to be honest with myself about what I can actually do.
The same level of honesty should be applied to Barclay and his disabilities, and that's exactly what they are because on numerous occasions his various mental disorders prevented him from performing his job. Starfleet must be pretty lenient though, because in any other situation with a command structure like Starfleet, he would have been fired.
And if Starfleet is supposed to be the best and the brightest, than they've lowered their standards, because he only meets half that qualification. He has the brains, but I would not want him representing the entire Federation, which is what every Starfleet officer is. In any real psych evaluation, he would have been red flagged during the academy if not before, and not been allowed to enter the academy or graduate.
Plus, I don't like that he's supposed to be a satirical composite of Trekies
, and how he just happens to possess many of the worst stereotypes about Star Trek fans. I'm surprised they didn't have him living in what amounts to some sort of basement of the Enterprise.