In TNG, Q announces that humans have piqued his interest, and he will be keeping an eye on them. Instantly every other storyline in TNG is reduced in importance. The only important story is what Q will decide to do. His existence diminishes everything else. Quite stupid.
I seem to recall that in the first season of the original series alone, there were at least two intelligences that would be continuing to keep an eye on humanity: the Metrons from "Arena" ("You are still half savage, but there is hope; we will contact you when we are ready") and the Organians from "Errand of Mercy." Of course they weren't as funny and engaging as John de Lancie's Q.
Once they had been admitted in the original series, the existence of such intelligences couldn't just be ignored. (Perhaps the Organians should have been part of TNG?) Writing Q as a super-Trelane with no restraints, whatever his effect on future episodes, was about the only way to go, if you consider that self-restraining types and parentally restrained types had already been done.