Yes, 'Past Tense' is really unique for DS9, it's the only two-parter that's completely standalone. It has some plotting problems - just about everything regarding time travel is horribly contrived, even by Star Trek's standards. But it's got a dead serious moral about how society treats the less fortunate, and some excellent character interaction with Sisko, Bashir, the thug, and the hostages. I particularly like how Sisko and Vin, the tough cop, played off each other. Vin starts out the model of bitter hostility, but behind all that he's a decent man. Obstensibly set in the more recent future, it's still an allegory for the present.
It's a very different turn for DS9, the closest thing it did like it was "Far Beyond the Stars", but it's a good diversion. I can't help but wonder if the plot would be better had Sisko beamed out as he was shot, thus 'dying': The original premise of the episode, IIRC. That would have been more contrived, but also with a better sense of martyrdom.
Season three's been pretty strong so far, except for 'Fascination', which as stated was a big 'meh' all round. I'm looking forward to 'Life Support', noting ironically that Bareil's last episode before he gets the axe had him acting like a complete buffoon.