I actually wrote kind of an essay about this recently, since DC's current attitude toward retcons annoys the hell out of me. It boils down to "I like retcons that build and hate ones that destroy." If you think of continuity like a museum, good retcons are the ones that turn displays into new rooms, and bad retcons are like ones that blow up rooms or seal them off.

So like, I like Speed Force as a retcon for the DC speedsters, since it takes a bunch of scattered origin-story displays and builds a new wing for them and adds a bunch of new stuff. It builds. Whereas Parallax being a yellow fear bug demolishes several whole wings of canon, from Hal's character development to everything that was unique about Kyle. It destroys. (And then remodels the entire Green Lantern section to be about itself and its siblings.) The retcon of the National Comics character Quicksilver into Max Mercury dusts off an abandoned artifact from storage and makes it into new room. That builds. The Top being responsible for all of the Rogues' reformations, ever, obliterates not only several characters' growth but disrespects the optimistic, progressive mood of decades of comics. That destroys.

But I do think that you ought to have a really good reason to do even a "building" retcon, rather than throwing them around willy-nilly whenever the mood takes you. You have to stick with them for awhile, too, otherwise all the breaking and building makes the museum structurally unstable and nobody wants to go inside anymore (to push a metaphor too far). In general, I prefer in-universe solutions to story problems, but sometimes that's not possible, and sometimes retcons can be good. But they're just usually lazy solutions to lazy writing instead, and oftentimes a better story could have come out of dealing with the fallout of bad writing instead of waving it away.

TL:DR: A retcon should add more to the story than it takes away.