In order to stay true to the look of the original Star Trek (1966) series, Jonathan West used 1960s style lenses and lighting techniques. He also used a 1960s film stock with a finer grain and 1960s style color saturation.
At the time it was made Ira Steven Behr commented that this was "probably the most expensive hour of episodic TV ever produced". The costs included the re-creation of the original sets, new costumes and props, new special effects shots, and residual payments to the original cast.
In the K-7 bar, O'Brien is convinced that Lt. Freeman is Captain Kirk. Lt. Freeman was played by actor and stunt man Paul Baxley, who also frequently served as William Shatner's stunt-double.
Walter Koenig (Pavel Chekov of Star Trek (1966)) visited the set and showed Colm Meaney and Alexander Siddig how to properly interact with the set pieces.
The Defiant's crew ask a reluctant Worf why 23rd century Klingons looked different that 24th century ones. Writer Ronald D. Moore knew of all the outlandish fan theories concerning this question, but he was afraid that every answer he would give would just be silly. So he chose to address the issue but dodge the answer, by having the crew questioning whether genetic engineering or a viral mutation were responsible, and Worf saying it is something they do not discuss with outsiders. These two reasons were later combined to ultimately answer the differences in Klingons' appearance in Affliction (2005)/Divergence (2005).
David Gerrold: the writer of The Trouble with Tribbles (1967) appears as a "redshirt", and picks up a Tribble in the Enterprise corridor. That Tribble is one of the originals from TOS.