During the autumn/winter of 2012 and early 2013 I worked with Kineo in their office on a couple of Drupal projects the main one being for the Civil Service. It's their learning portal site as has over 200,000 active users. So it's a high traffic site with multiple developers. As the project was started a couple of years ago it's in Drupal 6. Being a public sector project there is high emphasis on accessibility including development for older browsers such as IE6.
The project provided a number of interesting challenges and exposure to working on a multiple developer environment with Drupal using GIT source control. This project improved my module development and highlighted for me the benefits and pitfalls of using Drupal Features.