Let's talk fan-service done right. You want to appeal to old-guard and new fans by pulling in past characters; OK -- but do it with a good story, and with details which develop from the specific nature of the individual characters.
Colicos and Campbell played larger-than-life and comic foil, respectively, while Ansara played a more reticent, even tragic hero. All those roles have been deepened here, and it's not surprising to see three veteran actors still have the stuff to light up a screen. What's more notable is how well Terry Farrell keeps up with them, still firmly in character.
This must have been an awesome episode for the three guests but also Farrell. But also for us, the viewers; this is arguably the best episode of any Trek featuring guest stars from a previous Trek, because it's apparent that the writer(s) paid close attention to the original stories that featured these actors, and included details that show they knew as much as the most rabid of fans, if not more -- and used that info as legitimate part of the story (e.g., Kor's suggestion about the armory is exactly what Kirk did in "Errand of Mercy").
There are plenty of TOS or TNG or DS9 episodes which don't realize their real potential; that's just the nature of grind-em-out weekly TV. But here we have a sequel fully as good as the originals: big characters, believable plot, strong execution. Before this, DS9 had good episodes in its own style. With "Blood Oath" they proved they could also be good Trek in the old style, without losing their own. Highly recommended.