The key thing I got a bit wrong was high-traffic mostly static "content", vs high-traffic "very custom" which is a) what I was used to at envato and b) what I thought we had on our hands at goodfilms.

I think in a growing site with a lot of custom per-user content (like a social network) the extra complexity of a cache layer and managing expiry is more pain than it's worth while iterating the product quickly. If you're mostly a content site, it's definitely the #1 thing you should be doing.

Realising that we're both at the same time, depending on the user or page, means that sometimes the cache stuff is the right thing to do, and sometimes not. I was leaning too far towards not, and happy now with the balance we've picked.