If memory serves, the primary driver of reduced traffic to Digg 4.0 was the fundamental shift in how their submission mechanism worked, not in a change to their layout. I recall some kind of promotion mechanism where superusers gained some amount of control over their "front page" equivalent.

Also, unlike Digg, Reddit hasn't been bleeding members to another site for some time now, so there's nowhere for Reddit users to go if they become dissatisfied. Does anyone have any ideas about why that is? Reddit is not, fundamentally, a hard product to emulate, and yet I know of no site even remotely as popular nowadays (HN aside, of course). Slashdot, Fark, and sites like those all seem to have mostly been consumed by Reddit. Is it the strong network effects that Reddit has, or does Reddit's moderation system provide a unique way of hosting a community that no one has reproduced?