One of my favorite things about DS9 was the incredible attention to tiny details. Not to say it didn't happen in the other series, but DS9 did it the most. And a lot of it is just a throwaway line or even just a panel or a background alien.
Example: in RULES OF ACQUISITION, Pel shows Quark how the sands peas will end up tripling his profits. Almost two years later, in THE WAY OF THE WARRIOR, O'Brien and Bashir are in Quark's with O'Brien flipping them off his wrist, and Bashir asking specifically for those sand peas. Nice callback!
My point is DS9 is FILLED with such little callbacks, and I love that! I always thought of them as Easter Eggs for those of us who followed the show the entire way. As many as there were, the show could have been called DEEP EGG NINE.