Because the magic of the universe runs on Magic A Is Magic A, the wizard who has focused on curses has learned through Exact Words and Loophole Abuse to make a curse which is only a slight annoyance but can offer a huge boon to the one who bears it.
He's cursed to be the cleverest person in the world... but "cleverness" in this case means being able to see through every argument, and not being able to shut up, so that it really is a curse.
Alternately, he's cursed with a demonic form with Super Powers... that are preventing him from using his normal, more powerful Super Powers.
The superpowers would normally be a blessing, but With Great Power Comes Great Responsibility... Eventually, he has a nervous breakdown because no matter how hard he tries, he can't save everyone. This is the real curse, the burden of the mighty, the guilt of a hero. Consumed by his failures, he cries himself to sleep at night, alone in his tiny apartment, with no time for friends, too busy scrambling to save the world to hold a steady job.
He's cursed to be the cleverest person in the world... but "cleverness" in this case means being able to see through every argument, and not being able to shut up, so that it really is a curse... but he figures out how to get around the "never shut up" part, and thus, find his curse useful.
He's cursed with a demonic form of Super Powers that are preventing him from using his normal, more powerful Super Powers... but then he faces the enemy immune to his normal powers and finds out that he is vulnerable to his demonic powers.
Parodied: People start lining up to get cursed by the evil wizard in the hopes of getting wonderful side effects as a bonus.
Sometimes the curse of berserker rages causes problems, sometimes the cursed character can overcome the negative effects through willpower and use their curse-boosted physical abilities to act as a true hero.
Lovecraftian Superpower. Yeah, it's a superpower-but you have to say "If you are a minor, pregnant, may become pregnant, or have a heart condition, please leave the room" before you use it.
His powers hurt like crazy while in use.
Averted: It's a terrible curse. Full stop.
Enforced: "We can't have something really bad happen to the hero, so let's make the curse inadvertently beneficial".
Lampshaded: "Oh, one of THOSE curses". "Huh?" "He's saying that it actually benefits the person cursed".
Invoked: "If I play my cards right, I can trick the wizard into choosing a curse that isn't too terrible at all".
Exploited: The hero has a wizard in his party specifically to place awesome curses on him.
Defied: "You'll place a curse on him. And the curse must be a true curse, none of this 'blessing in disguise' nonsense!"
Discussed: "You do realize that some people would kill to have that curse placed on them, don't you?"
Conversed: "Why is that character complaining? He got Super Powers, and none of the usual downsides thereof".
The failure of the curse actually subverts the morality of the setting, as the threat of curses were all that kept people in line.
He is the cleverest person in the world, so everyone seems to him as children. He cannot find anyone who would understand him, doomed to be Surrounded by Idiots. It may additionally result in depression after overthinking the meaning of life etc.
Reconstructed: "With great power comes great responsibility", and responsibility is meant to be the "true" curse.