It really is unfortunate that Denise Crosby was unsatisfied in her role on The Next Generation and missed out, personally and financially, on the eventual monster success of the show - but her departure here in "Skin Of Evil" was a step forward for the show. Tasha Yar is simply not an interesting character - it was Roddenberry's attempt at another Uhura statement on women (in Yar's case, that women were just as strong and commanding as men) that really only contributed to all the stuck-up Star Trek finger-wagging at the late 1980s seen in season one. Crosby did an admirable job in the role but the character was weak on paper and the departure of that character was a good thing for the show, however dismissive it feels, as it leads to the promotions of and greater screen-times for actually likable and interesting characters of the show: Worf and Geordi.
As far as the actual "Skin Of Evil" episode goes, it is not bad: Troi crash lands on a planet and the Enterprise goes to rescue her but they find an evil tar-like being, Armus, who will not let them rescue her.
Some ideas eventually did not look as good as they must have on paper - the special effects are a huge let down, Riker's decent into the tarry pool of Armus is pretty lame, even Yar's demise is awkward (too quick and untimely for a supposed tough-as-nails security officer), and her final holographic farewells to the crew is marshmallowy lame and kind of dumb (do Starfleet crew members make sure that their farewell holograms are the most up to date as possible?). But the episode does include a great theme on violence, does include some good tension (Armus' cruel joke with Data is especially good), and Marina Sirtis gives her best performance of season one and one her best performances of the entire show. Overall I would say that "Skin Of Evil" is, while disappointing in many areas because of the potential it had to be great, a pretty good episode of the first season of Star Trek: The Next Generation if not only because of the importance of the passing of Tasha Yar.