For me, it was the numbers. I didn't really discover baseball until I was in college, in the 80s, when a roommate left a copy of that year's Bill James Baseball Abstract in the suite. I was smitten. It wasn't just the statistics - it was the analysis and the writing.
James understood that baseball statistics was about individual performance in a team context and knew how to separate the two. He knew how to make the numbers tell a story. It wasn't just playing games with numbers; it was bringing insight into the game to the numbers. Every article, every year, had something fresh and surprising, putting all those statistics together in a different way. Most importantly, he knew how to write.
James stopped writing the Abstract because he found that, while he still loved the game, he had run out of ways of surprising people. But it left me in an awful fix. At that time, there was virtually no free historical or current data available on a large scale. Current data was there, every Tuesday and Wednesday, on the USA Today sports page. I might be able to punch in a few numbers and come up with the predicted wins for the teams, or with the runs created for a few players. But other than that, there wasn't much out there.
What a difference a decade makes.
In true Army of Davids fashion, James also organized a fans' response to the Elias Bureau's hoarding of official stats. He developed an easy-to-learn, fairly-easy-to-use scoring system, and then helped organize a network of fans to score games and collect the stats on their own. This had the effect of breaking the Elias monopoly in stages, to the point where even Elias itself uses the Project Scoresheet scoring system.
There are now publicly available, real-time and historical databases online. There are publicly available free stats packages and databases to help analyze them. Fortunately, Baseball Hacks can help you locate, set up, and use parts one and two. You still have to provide the third ingredient--analytical skill--on your own, though.
The name sounds like a John Kruk at-bat, but in fact, it's a brisk and illuminating tutorial in how to load those public databases onto your own computer's database, and then how to access them for statistical analysis.