Terminus is odd in that there are four distinctly different storylines occurring at once, and none of them are directly related to the other. The Doctor and Kari are trying to stop Terminus from automatically jettisoning fuel, thus triggering the second Big Bang. (So long as you believe that such a routine procedure could result in the creation of the universe in the first place). Nyssa is struggling with Lazars disease and the Garm, while Olvir tries (poorly) to rescue her. Valgard wants to find The Doctor and Kari to settle a score between he and Eirak, the prize being leadership of the Vanir. And Tegan and Turlough waste most of their time in underground passages, popping up to the surface just long enough to try and find the entrance back into the TARDIS.
None of these storylines really work because none are important enough to base a four-part Doctor Who episode around. Which is supposed to be the main storyline here? All four vie for attention, all at the expense of each other. The end result is that when Nyssa suddenly decides to stay behind and help, it seems like it's coming out of the blue.
Nyssa was Peter Davison's favourite companion, but the two spend precious little time together during her final story. We're never given any hints at what a strong relationship it could have been. The lack of attention on Nyssa does not negatively affect her leaving scene, though, which is almost gut wrenching. The Doctor and Tegan are not so much sad about their friend leaving as they are worried for her safety. It raises the intensity of Nyssa's decision to stay on Terminus, and it is certainly one of the bravest choices made by a companion. These circumstances also increase the emotion in Nyssa's final scene, both from Sarah Sutton (who is lovely here) and a really forlorn Doctor, played with just the right amount of subtlety by Peter Davison.
Davison is often the best thing in his stories, and he copes fantastically well with the various issues that were going on behind the scenes at the time to deliver another strong performance. However, it was a troubled production like Terminus that possibly convinced Davison to make his next season of Doctor Who his last. And, out of all that was wrong with Terminus, that might be the worst thing of all to come out of it.