Great costumes and makeup also. It was a treat to see the Tellarites and Andorians.
I remember liking this one as a kid because we get to see a whole bunch of new aliens.
This really helped build on the concept that the Federation was not monolithic and was, in fact, a thriving, expanding - almost a "Wild West" feel at the frontiers, with the stodgy government-types trying to get a handle on the situation. Sometimes, it is difficult to separate Starfleet from "The Federation" - and this type of episode explored some of those distinctions (and more so than some of the cliche bureaucratic interference episodes - this one seemed very serious, from a diplomatic, political and, ultimately, a tactical situation.)