When he's not indulging his fantasies, he's good at figuring out what's causing strange mutations and break down in matter and the cause of strange apparitions in the transporters in "Realm of Fear." He forces himself to face his fear of the dreaded transport device in order to give a more detailed report of what he saw, all the while obsessing with the possibility of being infected with "transporter psychosis."
In "Genesis," he even has a new disease named after him (Barclay's Protomorphosis Syndrome) after contracting a flu in which Dr. Crusher treated with a shot of a T-cell to replace a dormant one. That T-cell contained DNA that causes human tissue to revert back to a much earlier stage in evolution.
He even gets a shot at being the most advanced human known in the Alpha Quadrant after being zapped by a probe on a transport shuttle in "The Nth Degree." With his accelerated I.Q., he figures out how to hook himself up to the ship's computer and gain complete control of it, sending the Enterprise to an unknown area of space and introducing the crew to a new advanced life form who has the ability to bring new species to them to say "hello."
The probe has other positive side effects, such as giving him new confidence which allows him to participate in plays without stumbling awkwardly through his lines and asking counselor Troi for a walk around the arboretum.
He also helps the newly emerged Professor Moriarty gain new life outside the confines of the holodeck in "Ship in a Bottle."
He approaches Zephrem Cochran like a school boy with a crush and nervously shakes his hand vigorously in the film "First Contact."
In one of the funniest moments in TNG ("Hollow Pursuits"), crew members give Barclay the nickname "Broccoli" behind his back. After the captain orders his crew to cease with the name-calling, he accidently mispronounces the Lieutenant's name to his face.
"I look forward to your report Mr. Broccoli.....Barclay."