What the biggest problems that the site has run into have been and how they were solved? It's way too easy to try and fix something that doesn't make sense at all only to discover that what appears nonsensical is actually the only fix for some subtle but nasty bug.
Going through the code and looking at anything that looks at all hard to understand and just asking "what does this do, why did you add it?"
Make sure you write down their responses - perhaps even comment them in the code so they are there when you need them. There is nothing more annoying than that feeling of "I know I was told about this..."
