I know, just a good natured poke :) Plus you could probably take that comment at face value - making use of web caching is definitely an important tool when building a large scale system.

Why is caching out of the window when logged in?

It's a page-level cache, and your view on a page depends on username, hidden submissions, point counts, etc.

votes, things to hide, etc all change the page and prevent serving everything wholesale out of cache

I'm not actually sure, just guessing...