Sorry to triple-post; I missed this one in my initial reading of the thread. The number of mentions of planets doesn't say much, since Romulans have interacted with the Federation to a considerably less extent than the Klingons. * Three of the five Trek series have had Klingon or half-Klingon protagonists, the Enterprise-D made multiple visits to Qo'Nos, the Federation was deeply involved in Klingon internal affairs and fought a year-long war with the Klingons, and there have been numerous recurring Klingon characters, including Dax's old friends who were present in two series. * The Romulans didn't interact nearly as much with the Federation. Federation-Romulan conflicts tended to be small and low key--the fight over that medical hologram-equipped ship, or the run-ins along the neutral zone--and the only Romulan characters I can think of who appeared for more than two episodes are Tomalak and the half-human Sela. If they don't feature much in the series, why would their worlds be mentioned?