If you are a developer who's interested in writing a plugin to WooCommerce, then you'll probably be interested in the following WooCommerce Plugin Development Guidelines:
Modern Retail has been using this guide as we write our first WooCommerce plugin, WooCommerce Integrator. This Integrator allows WooCommerce to connect to various point of sale (POS) and Enterprise Resource Planning (ERP) systems.
Of course you will also want to adhere to good WordPress and PHP Coding Standards:
One of the things we missed, which Matt Cohen caught, was some duplicate functionality with our logger. WooCommerce already includes a logging class which can be leveraged instead of writing your own.
Developing any sort of software typically involves making two parties happy, the development team reviewing your code and the folks in marketing. Make sure you are using the WooCommerce brand correctly by following their Style Guide:
Developing a plugin for WooCommerce has been relatively painless, and we hope our POS & ERP Integrator is the first of many plugins we'll be writing for WooCommerce.