the story: The station inadvertently becomes home to a Jem'Hadar youth.
what it's all about: A bit like the Next Generation classic "I, Borg," "The Abandoned" unexpectedly introduces sympathy for the devil via an innocent. But the comparisons end there. Where "I, Borg" was a stark contrast to every other Borg appearance (until Voyager's Seven of Nine, of course), "Abandoned" actually introduces a lot of interesting wrinkles, many of which would be improved upon later, to the concept of the Dominion.
This was still early in the existence of the Dominion in the series, mind you, and so "Abandoned" is also kind of a cheat and/or placeholder until someone decided just how far the story would really go. Where "I, Borg" felt momentous (even if followed up poorly in the "Descent" two-parter), "Abandoned" in retrospect feels like it doesn't really belong in the greater arc except as a matter of convenience to establish a few basic elements of Dominion lore.
As the first manifestation of the Dominion, the Jem'Hadar were immediately presented as formidable warriors, the first true rivals to the Klingons for this class in Star Trek lore (this wouldn't go unnoticed later, trust me), but they were also instantly presented as virtually interchangeable, guided in all things either by the Vorta or Founders. In fact, "Abandoned" begins to establish just how guided, whether by genetic engineering or drug addiction (an interesting social commentary I'm not sure the series ever really did enough with).
But the real benefit of the whole experience is what all this has to do with Odo. As established in the two-part "Search" season opener, Odo's people are the Founders, and as such anyone from the Dominion has to defer to Odo or risk offending their "gods." (Deep Space Nine's commentaries on religion, begun with the Bajorans, are thus ratcheted up an impressive degree.) The rebellious Jem'Hadar youth can only be contained in his violent impulses by Odo, which actually makes this more of an Odo episode than an exploration of the Dominion.
Where it fails is, again, because it exists in a vacuum, because the Jem'Hadar youth really has no useful direction to go at the end of the episode except to disappear and never be seen again (which is actually a shame, and one of the great missed opportunities of a series that had few of them). Why this happened in the third season rather than the first or second is only because the creators only thought of it then. Otherwise it's an almost embarrassingly random event that's otherwise not indicative of how Deep Space Nine tended to work.
The perhaps better element of the episode is the B-story, in which Jake begins to exert himself by introducing Sisko to his risqué Bajoran girlfriend, who works at Quark's bar. It's a hilariously awkward subplot that shows how much better the series had gotten at exploring this unique father-and-son duo, and actually does everything the main story did but, well, better. A good setup for the later-in-the-season classic "Explorers."
franchise- Superficial similarities to "I, Borg" not worth totally acknowledging.
- series - It does establish some useful things about the Dominion.
- character - It's a good way to further establish Odo's odd relationship with the Dominion, too.
essential- But all of this would be done better later ("Treachery, Faith, and the Great River").