Isn't every single subreddit cachable?
Only the starting page isn't 100%, but I'm sure that many users have the same combination of subreddits (e.g. the default ones).
Would be interesting to see if they have certain "user classes". Something like:
For logged in users, you're still going to need the voting status for the current user on every single submission as well as that submission's hidden status for the current user to decide whether or not a submission should be displayed in the listing.
I believe they almost never hit the DB directly, so these are probably recached immediately (or submitted to both the cache and the DB at the same time), but that still means quite a lot of traffic.
USERCLASS = SHA1(SUBREDDIT1 XOR SUBREDDIT2 XOR SUBREDDIT3)