Structured overview on the things to keep in mind while writing your code. I think this topic and the length of the talk on this is very dependent on the audience. Personally I think most of the stuff should be basic knowledge of any programmer. It would've been nice to have some more info about the Big-Oh notation and algorithm complexity. I have the feeling PHP developers care less about that than, for example, Java developers.
Big win for me was that I finally got profiling and debugging working in my IDE in the practical part.
I didn't know much about Varnish so this talk was definitely a must for me. The talk itself had everything in it to understand how Varnish works, the overview and stepping through it is exactly what most talks about tools are often lacking. I found it to be a huge benefit for understanding the workings. The VCL and ESI code snippets finished it off and this talk has given me enough offset to try and poke around with it.