I'm not that familiar with Digg (it's the same as Reddit though, right?), but two major things occur to me:
1) I'm much less likely to vote a answer/question up/down on SO than I am at Reddit. On SO, if I'm not asking or answering, I'm rarely causing any writes to the data store. On Reddit, I vote on most of what I look at. I could see this having a huge impact.
2) Obviously Reddit can do some caching, but I think SO can cache much larger pieces of data. As far as I know, everyone who goes to the main page of SO sees the exact same list of questions. On Reddit, each subreddit's top items can be cached, but they are mixed differently for each user.
1) I'm much less likely to vote a answer/question up/down on SO than I am at Reddit. On SO, if I'm not asking or answering, I'm rarely causing any writes to the data store. On Reddit, I vote on most of what I look at. I could see this having a huge impact.
2) Obviously Reddit can do some caching, but I think SO can cache much larger pieces of data. As far as I know, everyone who goes to the main page of SO sees the exact same list of questions. On Reddit, each subreddit's top items can be cached, but they are mixed differently for each user.