- Cache for unlogged users, have a less dynamic front page (let's say 10s cache), that would help a lot

- Cache of most popular topics (again, even a 10s invalidation time) would help

For the logged users you could do something more dynamic, for example, cache the front-page snippets of the main subs + add the ones for the user. And again cache that

(unless the 'new reddit' makes this harder, to which I would say - well deserved! ;) )