I see traditional email and newsgroup clients missing a bit from Krinkle's list. Subthreading works perfectly fine in Thunderbird (but even in Outlook Express!). Indenting is the one characteristic found in all wiki conversations: subthreading can't be discarded based on gut feelings. In my experience shepherding thousands of it.wiki conversations (mostly on their own subpages), the biggest issue are offtopic and personal tangents, hence the most important technical feature in wikitext is that I'm able to easily tell apart, link and move subthreads.
Nemo