^On paper, he was exactly what you say. But the execution left much to be desired. We weren't seeing a Vulcan laugh. We were seeing a jovial human who just happened to be wearing pointed ears.
Think about the various times in TOS when Spock's emotions were let loose. There was always something creepy and disturbing about it. Perhaps because we were used to seeing Nimoy acting emotionally restrained. But that's how they should have cast Sybok--get somebody who's more introverted, and make them act extroverted. In casting somebody who came off as completely, naturally extroverted, they didn't sell the point.
A more intriguing way to portray a Vulcan who embraces his emotions would be to have him embrace negative emotions as well as positive ones. For the "villain" of the story, Sybok was unusually likeable. Imagine if they'd given him a pronounced negative side--had him flying off in rages with little provocation, that sort of thing. Show us that there's a reason that the Vulcans are so big about restraining their emotions, as TOS established.