well, that particular question came up because Scoop doesn't handle really deep threads very well. Loads of comments on a page does still slow it down, but because Scoop builds up the threads recursively it's the deep threads that are the killer - they eat up memory like crazy.
This is pure speculation, but I think slashdot manages as many users as it does because it archives stuff very quickly, thus getting it out of the db and keeping the size down, and because I heard that they'd ported it to Oracle or some similar seriously heavy-duty database. Scoop can already do the archiving, and there is work underway to allow for different database backends. (I know the work has been underway for ages, and no I don't know when it'll be done. I don't know what's involved in a job like that.)
Slash gets around the long/deep threads issue with its comment paging, which has some advantages and some issues (I believe I heard somebody call it "braindead" in how it handles threads longer than the number of comments allowed on a page...)
I think the idea presented in the article you link to about cutting off really deep threads with a "continued..." link is a good one - that alone will make a big difference in how Scoop scales.
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Discuss the art and craft of writing
