Keiko's not my favorite character by a long shot, but I found her and O'brien's relationship to be one of the more believable relationships.
As for Jadzi and Worf, I dunno. Too much just seemed to be comic relief (by them ending up with broken bones, etc.) and a way to showcase Jadzia's Klingonphilia. I didn't really mind it though.
Quark and Brunt
Some of the Brunt episodes were god damn stupid and cartoonish.
On a similar note, Moogie and Rom.
i don't dislike any of those characters, but I agree that things got a bit too goofy and cartoony at times when they focused too much on them.