That's really informative, thanks!
I assume when you recommend I redirect discontinued pages to the associated category page you mean redirecting via a 301?
Sorry I probably wasn't very clear on my final point about my concern about having too many 301's. My main concern about building tens of thousands of 301's into my site was that it would potentially have an impact on how quick my pages loaded for users (and crawlbot) since it would have to check through so many 301 rules. Have you experienced this as a reason not to use mass implementation of 301s?