Genius Bonus: The Doctor distracts a guard by discussing semiotics with him. The real joke... the dialogue came verbatim from a semiotics text examining Doctor Who. And, the Doctor's line is semiotics-jargon for something like "The less relevant an in-joke is to the story, the greater its cultural significance". Particularly impressive for a story which came out way back in 1987, before such post-modern humor appeared everywhere.
Les Yay: Between Ace and Mel. Especially when Ace's first response to Mel falling over and being knocked out is to do what is probably meant to be feeling for a skull fracture, but looks more like stroking her hair. It's more blatant in the novelisation, where Ace and Mel descend an ice face using mountain climbing gear rather than the pack ladder used in the serial and this exchange takes place:
'I've never done this before,' admitted Mel. 'You must have done it lots of times.' ‘Well, not lots of times…’ Ace was avoiding Mel’s eyes. ‘How many times?’ asked Mel suspiciously. ‘This’ll be the first.’ Ace looked up guiltily. ‘But I’ve seen them do it on telly, and it’s easy!’ Mel looked at Ace. The safety harness Ace was wearing didn’t look quite right. ‘I think you’ve got that harness on upside-down. I think those tight straps are supposed to go between your legs.’ Ace looked down, and giggled. ‘It’s a good job I’m not a boy!’ she laughed. Mel smiled - and then she began to laugh as well. This wasn’t going to be a bit like they always showed it on telly!
Moral Event Horizon: Kane has the tourists, passers-through, and residents herded into a spacecraft and blows it to Kingdom Come.