Features
If you were to pick a single thing to start with, I'd start with these. Then again, if you were to pick a handful of things to read, I'd still recommend these. Going beyond blogging and banter, these posts capture concepts and ideas that may be incredibly useful to the general populace. Not that other entries aren't fun, but you know what I mean. :)
Fowler and Transactions in PHP
In a game-like economy, you need a consistent way to keep track of someone's assets. This is made even worse with a PHP based website, introducing a stateless system. With one user who can have multiple windows open at the same time and performing different actions, steps need to be taken to ensure account integrity. Learn about Fowler's implementation of the Account and Posting Rules patterns, and their applicability to Gaia Online.
Crazy JavaScript Antics
Legacy code can cause all sorts of problems. From old form controls, outdated JS calls, and crufty HTML 4.0 comes a unique set of problems. To anyone who's had to work with code from "back then"...