I've always enjoyed not just gaming, but analyzing the games themselves as well. Back when I had oodles of spare time, I really loved playing Populous: The Beginning, and I got especially adept at the game, so I was especially excited when someone made a fan mod of the game, Populous: Age of Chaos. It was a brutal mod, and the only guide I could find gave up on level 4. So I decided to sit down and analyze the game, and make my own guide.
It was an obvious first attempt at not just the guide itself, but the website that hosted it. The writing could use a lot of editing, and design is ridiculously outdated (admittedly, it was going out of fashion even when I made it), and the construction was ridiculously inefficient. However, it was well-organized, with clear instructions, and well-documented pictures, and ended up being useful for a lot of other players. I'm proud of it, within a historical context.
Having that kind of a history with game design documentation has been an amazing boon towards understanding and creating my own games.