I think the Odo/Quark relationship was something they had right from the beginning, and it's often a highlight of earlier episodes.
Whilst the concept itself of a station-wide virus was already a Trek cliche by this point, I think that there was a lot of nice character work in Babel. I particularly like Kira's desperation in trying to make things right, and also Quark enjoying himself in Ops when there's just him and Odo left.