None.

The CSO is pretty dire. And while the giant maggots are a design triumph up close, the long shots where real ones are super-imposed onto shots of Bessie et al don't convince for a second. And neither does the giant fly. Doctor Who has had several goes at giant flies, and remarkably has never topped its first attempt (in Planet of Giants).

Otherwise, I'm not sure the detour to Metebelis 3 is really necessary. I don't mean so much in story terms, but in in dramatic terms it's something of a cul-de-sac. And I'm not too sure about B.O.S.S. I'm not sure what the story is trying to say about technology. It's neither as threatening nor as vivid as WOTAN, although the visual representation is an improvement.

This is a great story. But I don't consider it one of Pert's very best, and it might require a re-watch to discover quite why that is.

Like with Hartnell in The Three Doctors, I understand that Elgin's abrupt disappearance from the plot after it really seemed like they were building to something with him was due to the actor's health issues, so I can't entirely blame the production team for it, but it is really irritating that they don't even try and come up with an explanation or even an acknowledgement of what happened to him. They already had to rewrite his scenes in episode five for a new character, so why not just throw in a single line or something explaining his absence? Aside from that and some REALLY bad CSO work, I can't really think of any major flaws for this one. Really damn good one the whole.